Do Not Write READNOIS FITS Keyword In a Noiseless File

[noqsi]

Currently, SPyFFI writes the READNOIS FITS Keyword even if it does not simulate readout noise.

SPyFFI should not simulate readout noise, nor should it write this FITS Keyword to the header.

Downstream in HTTM we are going to throw a warning if the user tries to process a file with this forbidden keyword. See TESScience/httm#25 for more details.