APE: Fix basefield handling
This MR introduces a continuous expansion for the APESolvers basefields.
This fixes a bug where interpolated fields cause the solver to diverge because currently, only discontinuities of the perturbation fields are handled in the rieman solvers. Forcing the basefield to be continuous evades this problem, enables smoothing (in a future MR) and increases the overall stability.
This also means we have to go back to the constant c formulation which should not cause any issues since the old implementation was quite unstable anyway.
This also adds the ability to compute the CFL number to the APESolver and adds the sourceterm to the fields written to fld files.