Replace NetCDF with a native "ff native" format #245

Collecting the thread from #154, #175, #170, #171, #207 and #208: drop NetCDF from the core, with an external tool converting NetCDF/global data into a compact native format that can also store state (fronts, atmap, data).

If useful as prep, I can move the 7 FF_NO_NETCDF #ifdefs into per-file stubs selected by CMake, so the new reader plugs in cleanly. Only if you want it.