Re-factoring of Dirichlet boundary conditions

In the parameter file we renamed grid/bc to grid/dirichlet_bc for clarity.

Moreover, the entries must now be boolean (True if hom. Dirichlet are to be applied). It is possible to set dirichlet_bc = None, in which case the boundary operators in Derham.__init__ are set as IdentityOperators.

Further changes:

• Renamed Projector to PolarCommutingProjector.
• All input files in io/inp/tests have been removed.
• Renamed struphy/psydac_api to struphy/feec
• Added the options --short-help, --fluid, --kinetic, --hybrid, --toy to the command line
• Started to add stuff to geom_projectors.py, but everything is commented out for the moment. The class PolarCommutingProjector() is unchanged.
• Expose dofs_extraction_ops in Derham.