Refactor subset checking of MultiIndexSet instances.

The method is_sub_index_set_of() of the MultiIndexSet class is now renamed to is_subset() for brevity and consistency with Python built-in sets.

The instance of the class now supports also the operator <= for subset checking. By default via the operator, if there's a different in spatial dimension, the set with a lower dimension is expanded. This behavior can be changed via a parameter in the method call.

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

Some additional changes include updating the test suite to include testing the subset checking.