Update python Interface for DirichletValues

[henrij22]

The python interface for dirichlet values as far as I can see doesn't allow to fix DOFs, that are not part of the boundary .
Normally I would have assumed that

ikarus/ikarus/python/dirichletvalues/dirichletvalues.hh

Line 108 in 081099d

cls.def("fixDOFs",

could fix other values, but this forwards to

 self.fixBoundaryDOFs(f);

This is in contrast to the interface that the C++ class has, where

ikarus/ikarus/utils/dirichletvalues.hh

Line 95 in 081099d

void fixDOFs(F&& f) {

does somthing else.

• Change interface to be more flexible
• Check if python bindings for subspacebasis are working
• Write a dirichletvalues test

Update 01.07.24

Most of this is fixed with #305 , only fixing non-boundary-dofs is not yet adressed properly

[henrij22]

This is obsolete with #305