Fix Issue #100: Change poly_degree to be a read-only property
- The attribute
poly_degree
of aMultiIndexSet
instance is now a read-only property. This value is inferred from the given exponents and lp-degree and will be unsafe to change from the outside once the instance has been created.
This MR should resolve Issue #100 (closed) and is part of the MultiIndexSet
class refactoring (Issue #99 (closed)).