CameraReadout.to_table is not FITS compatible

[kosack]

CameraReadout.to_table().write("readout.fits") creates a HIERARC keyword in the output FITS file, since the REF_WIDTH header is > 8 characters. Suggest changing it to REFWIDTH. Or we just deal with the strange data model (it's fine for other formats like HDF5 and ECSV, just not FITS).

[maxnoe]

Let's use REFWIDTH. I think it was meant to be fits compatible but miscounted, otherwise a more explicit name would have been appropriate.