Arity as member value

Currently arity is a member function of different types such as MDIs, Sets, etc. It poses a problem for structures that have an empty value such as:
- Sets
- PWMaps

which, for example, returns {} as the complement of {}. This wasn't a problem in older versions because the complement operation was a helper function to calculate the difference of sets, and thus was private. This have changed, so another client of the library might get an invalid result because of this.

The number of dimensions of the set is determined by the evaluator, so the idea is to:

1. Have arity as a member value of Sets and PWMaps.
2. Add an argument in their corresponding factories constructors.
3. The factory initializes the correct arity while creating elements.