Add abstract method copy to base class FemSpace

[yguclu]

This is especially useful when the H^1-conforming scalar space is extracted from the mapping, and its dtype (which should be called codomain_type) is changed from float to complex in order to accommodate complex-valued fields.

TO DO:

• Add abstract method copy(self, dtype=None), which changes the dtype if required, to base class FemSpace.
• Implement method in concrete classes SplineSpace, TensorFemSpace, VectorFemSpace, and ProductFemSpace.
• Remove function change_dtype from psydac.api.discretization, as this was used to replace the vector space of coefficients in place (very dangerous).
• Fix any bugs exposed by the previous changes.