force_complex_fields=True and real/imag for 2d simulations with kz ≠ 0 are incompatible

[stevengj]

As discussed here: https://github.com/NanoComp/meep/pull/3169/changes#r2933490015

Two options:

1. If you are using the real/imag trick and $k_z=0$, throw an error if the user passes force_complex_fields=True — this should certainly be an error if the user explicitly passes both options
2. If the user passes force_complex_fields=True, disable the real/imag trick (or at least the default).