Support for Periodic boundary conditions

[RemDelaporteMathurin]

It would be nice to support periodic boundary conditions.

It is possible in legacy fenics, I would assume it is also possible in fenicsx.

In legacy fenics, these BCs are not treated as DirichletBCs but added as a constraint to the functionspace. https://fenicsproject.org/olddocs/dolfin/1.4.0/python/demo/documented/periodic/python/documentation.html

[RemDelaporteMathurin]

Adding the fenicsx flag to this since it's relevant for FESTIM2