Add disjoint checking of MultiIndexSet instances.

The method is_disjoint() is added for the MultiIndexSet class to carry out disjoint check between two instances of the class. There is no equivalent operator for this method. By default, if there is a difference in the spatial dimension, there is no dimension expansion and an exception is raised. This behavior can be changed via the expand_dim parameter.

This MR, along with MRs !98 (merged), !99 (merged), !100 (merged), !101 (merged), should resolve Issue #129 (closed).

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