Union operation is now supported for MultiIndexSet instances.

The union between instances of MultiIndexSet may now be created using either a method call (MultiIndexSet.union()) or an operator (|). The corresponding inplace operation is also supported via the inplace parameter (for the method call) or the inplace operator (|=).

The resulting union instance has the union set of the exponents. By convention, if two multi-index set operands have different spatial dimensions, the resulting union set has the largest dimension of the two operands. Furthermore, if two multi-index set operands have different lp-degrees, the resulting union set has the largest lp-degree of the two operands.

This MR should resolve Issue #124 (closed) and related to Issue #99 (closed).

Additional changes:

• The test suite is extended to test the behaviors of both the utility function and the method for taking the union of MultiIndexSetinstances.
• The additional series of tests is now grouped in a class for organization purposes.
• A new short How-To Guide to demonstrate taking the union of MultiIndexSet instances is added.