Add support for equality checking via the == operator for MultiIndexSet instances.

An implementation of __eq__() now allows for checking the equality in value between two instances of MultiIndexSet via the == operator (resp. the != operator). Two instances of MultiIndexSet are equal in value if and only if:

• the underlying exponents of the instances are equal and
• the lp-degree of the instances are equal.

The second condition means that even if two instances have the same underlying exponents, the instances are not equal if the lp-degree is not equal. Furthermore, if both the exponents and lp-degree are equal, then it is safe to say that both the spatial dimension and poly. degree are also equal; therefore, they are not explicitly checked.

This MR should resolve Issue #107 (closed) and related to Issue #99 (closed).

Some additional changes:

• The test suite is extended to cover the behavior of equality checking.
• The How-To Guide is updated with a demonstration of how to carry out equality in value checking between instances of MultiIndexSet.