Use numpy-like descriptions for scalar type in command line

[chrisrichardson]

Instead of using C types, it would be neater to use numpy types for the scalar type, when needed.
e.g.
ffcx --scalar_type=complex128 Form.py rather than ffcx --scalar_type="double _Complex" Form.py

The translation to implementation scalar type would take place in the "C" language implementation part.

[chrisrichardson]

This will require changes in dolfinx and anywhere that calls ffcx with scalar type.

[chrisrichardson]

Note that some languages have sufficient templating to defer the choice of scalar. E.g. c++ or numba

[chrisrichardson]

Fixed in PR #647