Add proper subset checking of MultiIndexSet instances.

The method is_propsubset() is added for the MultiIndexSet class to carry out proper subset checking. The instance of the class now also supports the operator < for proper subset checking. By default via operator, if there's a different in the spatial dimension, the set with a lower dimension is expanded. This behavior can be changed via the method call.

This MR should partially resolve Issue #129 (closed).

Some additional changes include updating the test suite to include testing for proper subset and the How-To Guide on set operations involving MultiIndexSet instances.