Refactor the Grid class

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

• Clarify the interface of the default constructor of the Grid class (see Issue #71 (closed); resolved via MR !125 (merged))
• Clarify the interface of all other constructors of the Grid class (see Issue #149 (closed); resolved via MR !126 (merged))
• Remove generating_values as a property of Grid (see Issue #148 (closed); resolved via MR !124 (merged))
• Verify that an array of generating points has unique values per column (see Issue #152 (closed); resolved via MR !128 (merged))
• Refactor expand_dim() to have consistent behavior when there is no generating function defined (see Issue #151 (closed); resolved via MR !127 (merged))
• Update the docstring of the class
• Consider renaming generator in from_generator() class method (resolved via MR !126 (merged))
• Implement apply_func() a method to evaluate a given function on the unisolvent nodes (will not be implemented in favor of __call__(). See Issue #153 (closed); resolved via MR !130 (merged))
• Refactor make_complete() according to the changes in MultiIndexSet class (see Issue #157 (closed); resolved via MR !134 (merged))
• Refactor add_points() and decide if this is still necessary (the method is renamed to add_exponents(); see Issue #158 (closed); resolved via MR !136 (merged))
• Add the method make_downward_closed() (see Issue #156 (closed); resolved via MR !135 (merged))
• Refactor _gen_unisolvent_nodes() function
• Refactor get_points_from_values() and decide if the -1 multiplication is still necessary
• Think about how to have grid points on different domain other than [-1, 1]. Should it be implemented at all? Should it be implemented here?
• Consider making Leja ordering of generating points optional.
• Add support for instances multiplication (see Issue #154 (closed); resolved via MR !132 (merged))
• Add support for instances addition (see Issue #155 (closed); resolved via MR !133 (merged))
• Create a series of How-To Guides that demonstrates basic usages, constructions, and capabilities.
  • Factory methods (resolved via MR !126 (merged))
  • make_complete() (resolved via MR !134 (merged))
  • make_downward_closed() (resolved via MR !135 (merged))
  • add_points() (renamed to add_exponents(), see Issue #158 (closed); resolved via MR !136 (merged))
  • apply_func() (removed in favor of __call__, see Issue #153 (closed), resolved via MR !130 (merged))
  • Grid multiplication (resolved via MR !132 (merged))
  • Grid addition (resolved via MR !133 (merged))