Introduce max_exponent and max_exponents properties in MultiIndexSet

While an instance of the MultiIndexSet has already the poly_degree property (whose definition depends on lp_degree), sometimes it is also important to know the maximum exponents (for each dimension) and the maximum exponent (for all dimensions). For instance, the poly_degree of a Grid is associated with maximum exponent instead of multi_index.poly_degree because it is defined as the maximum of all one-dimensional polynomial degrees.

These two quantities can be found by using np.max():

max_exponents = np.max(multi_index.exponents, axis=0)
max_exponent = np.max(multi_index.exponents)

but if it would more convenient to access these quantities via properties:

max_exponents = multi_index.max_exponents
max_exponent = multi_index.max_exponent

Adding these properties doesn't change the overall functionality of the codebase.