Change poly_degree attribute of MultiIndexSet to be a read-only property

Currently the attribute poly_degree of a MultiIndexSet instance is simply an instance attribute; after an instance has been created (either via the default constructor or from_degree()) the attribute can be arbitrarily changed from the outside.

Exposing the attribute is potentially dangerous for the integrity of the instance. poly_degree should only be changed via internal mechanism of the class, e.g., adding a set of exponents may change the value of the polynomial degree of a multi-index set.

To protect the attribute, define a corresponding private attribute (i.e., _poly_degree) and implement read-only property to access the value of poly_degree.