Fix Issue #66 - Inconsistent poly-degree in MultiIndexSet

This merge request depends on the merging of !35 (merged) as there was a separate bug (Issue #65 (closed)) on the routine to compute the polynomial degree given a set of exponents and lp-degree itself.

• The poly-degree of a given exponents and lp-degree is now correctly computed and assigned when creating an instance of MultiIndexSet.
• The test suite has been extended.