Add support for in-place multiplication for MultiIndexSet.

The product/multiplication between instances of MultiIndexSet may now be carried out in-place either via a method call (multiply()) with the inplace parameter set to True or via the in-place multiplication operator (*=).

This MR should resolve Issue #125 (closed) and an extension of the MR that resolved Issue #119 (closed).

Additional changes:

• The method multiply() also supports out-place multiplication by setting the parameter inplace to False (which is the default).
• The test suite is extended to test the behaviors of in-place multiplication between MultiIndexSet.
• The test suite related to the multiplication of MultiIndexSet and arrays of multi-indices is now grouped using classes for organization purposes.
• The How-To Guide to demonstrate taking the product of MultiIndexSet instances is updated with a section on in-place product.