🧹 Re-factor FVM lowered cell implementation and shared state

[thorstenhater]

This is a clean-up operation on fvm_lowered_cell_impl.hpp and {cpu, gpu}/shared_state.{hpp, cpp}.

• Get rid of shared_state specifics in fvm_lowered_cell_impl and push these down to shared_state
• Present more of an interface layer towards fvm_lowered_cell from shared_state
• De-duplicate methods across CPU/GPU implementations by adding shared_state_base
• Align interfaces of GPU/CPU solvers.

[thorstenhater]

bors try

[bors]

try

Timed out.

[thorstenhater]

Well, forgot to push the actual changes.

[boeschf]

thanks for refactoring. Looks much cleaner now.