Add maximum exponent(s) properties to MultiIndexSet.

Two new properties have been added to the MultiIndexSet class. The max_exponent property retrieves the maximum exponent across all dimensions, while max_exponents gives an array of the maximum exponents for individual dimensions.

These properties may be useful to look into especially for non-complete multi-index sets where the values may differ from the polynomial degree (i.e., poly_degree) of the multi-index set.

The related tests have been updated to test the new changes.

This MR should resolve Issue #150 (closed).