Refactor the MultiIndexSet class

This is the parent issue for the refactoring of MultiIndexSet. The following changes are proposed (the list may be extended):

• Fix edge cases when inferring the polynomial degree given multi-index set and lp-degree (see Issue #97 (closed); fixed via MR !63 (merged))
• Remove the setter for lp_degree property of the MultiIndexSet (see Issue #98 (closed); fixed via MR !66 (merged))
• Fix memory issue when adding exponents after the property is_complete has been queried once for large sparse problems (see Issue #81 (closed); fixed via MR !77 (merged))
• Make explicit the default value for the lp_degree parameter of the from_degree() class method (see Issue #76 (closed); fixed via MR !64 (merged))
• Change the instance attribute poly_degree to a read-only property (see Issue #100 (closed); fixed via MR !65 (merged))
• Consider removing the default value for the lp_degree parameter in the default constructor (see Issue #37 (closed); fixed via MR !64 (merged))
• Consider allowing values of non int type as the polynomial degree for the from_degree() constructor as long as the value remains reasonable (see Issue #101 (closed); resolved via MR !86 (merged))
• Consider allowing values of non int type as the spatial dimension for the from_degree() constructor as long as the value remains reasonable (see Issue #77 (closed); resolved via MR !85 (merged))
• Consider removing exponents_completed property (see Issue #104 (closed); resolved via MR !104 (merged))
• Refactor the _new_instance_if_necessary() method, especially if the exponents_completed property is removed (resolved via MR !104 (merged))
• Rename the is_complete property to is_downward_closed, or keep both but distinguish them (see Issue #105 (closed); resolved via MR !83 (merged))
• Consider adding make_downward_closed() method (see Issue #105 (closed) and Issue #123 (closed); resolved via MR !84 (merged))
• Consider adding an inplace parameter for the make_complete() method (see Issue #103 (closed); resolved via MR !69 (merged))
• Consider adding an inplace parameter for the add_exponents() method (see Issue #117 (closed); resolved via MR !77 (merged))
• Consider adding an inplace parameter for the expand_dim() method (see Issue #118 (closed); resolved via MR !78 (merged))
• Fix the documentation issue of the add_exponents() method (see Issue #117 (closed); fixed via MR !77 (merged))
• Consider having an __eq__ dunder method for MultiIndexSet instances (see Issue #107 (closed); resolved via MR !88 (merged))
  • Two MultiIndexSet instances are equal when both the exponents and the lp-degrees are equal.
• Clarify the behavior of copy.copy() and copy.deepcopy() for MultiIndexSet (see Issue #106 (closed); resolved via MR !97 (merged))
• the dunder method __repr__() should provide an unambiguous string representation of the instance (see Issue #133 (closed); resolved via MR !106 (merged))
  • The information provided by __repr__() can, in principle, be used to recreate the instance using the default constructor.
  • This means the lp_degree information should be included in the representation and be distinguished from __str__()
• Define a dunder method for multiplying two instances of MultiIndexSet (see Issue #119 (closed); resolved via MR !80 (merged))
• Define a dunder method for multiplying two instances of MultiIndexSet inplace (see Issue #125 (closed); resolved via MR !94 (merged))
• Define a dunder method for creating a union of two instances of MultiIndexSet (see Issue #124 (closed); resolved via MR !89 (merged))
• Define the special method __contains__ so individual multi-indices may be checked for containment (see Issue #128 (closed); resolved via MR !103 (merged))
• Define method/operator for subset check (see Issue #129 (closed); resolved via MR !98 (merged))
• Define method/operator for proper subset check (see Issue #129 (closed); resolved via MR !99 (merged))
• Define method/operator for superset check (see Issue #129 (closed); resolved via MR !100 (merged))
• Define method/operator for proper superset check (see Issue #129 (closed); resolved via MR !101 (merged))
• Define method/operator for disjoint check (see Issue #129 (closed); resolved via MR !102 (merged))
• Define an empty MultiIndexSet instance (see Issue #132 (closed); resolved by MR !96 (merged))
• The default constructor of MultiIndexSet should accept lexicographically unsorted and repeat exponents (see Issue #120 (closed); resolved by MR !79 (merged))
• Refactor the implementation to find the position of an index within a multi-index set; use binary instead of linear search (see Issue #121 (closed); resolved via MR !81 (merged))
• Refactor the generator to yield missing multi-indices with respect to downward-closedness to avoid duplicate terms (see Issue #122 (closed))
• Clean up the in-code documentation (see Issue #134 (closed); resolved via MR !107 (merged))

When applicable, multiple merge requests will be made for the proposed changes; a separate issue may be created in order to elaborate the problem further.