Clean up and update the available set operations associated with MultiIndexSet instances

The current implementation of MultiIndex supports three set operations:

• union: create a union from two multi-index sets.
• is_sub_index_set_of: Check if a given multi-index set is a subset of another.
• is_super_index_set_of: Check if a given multi-index set is a superset of another.

To conform with Python set operations, we are going to update these methods with corresponding operators along with a couple of new methods and operators. For now, we are going to focus on comparison operations. Let mi_1 and mi_2 be two instances of MultiIndexSet.

Operation	Method	Operator	Remark	Resolution
Subset	mi_1.is_subset(mi_2)	<=	renamed from is_sub_index_set_of	MR !98 (merged)
Proper subset	mi_1.is_propsubset(mi_2)	<	New	MR !99 (merged)
Superset	mi_1.is_superset(mi_2)	>=	renamed from is_super_index_set_of	MR !100 (merged)
Proper superset	mi_1.is_propsuperset(mi_2)	>	New	MR !101 (merged)
Disjoint check	mi_1.is_disjoint(mi_2)	N/A	New	MR !102 (merged)

This issue is related to the refactoring of the MultiIndexSet class (Issue #99 (closed)). Note that some of these operations are already listed as TODOs in the source code.

Several other obvious set operations like intersection and difference are for now deferred because we need to define an empty MultiIndexSet to support those operations.