Sanitize FITS headers before sending to fitsio.

The camera actors are pretty careful about catching errors while constructing headers, but we should run a pass over the header to pick up any stray problems we can think of: fitsio evidently blows up without writing the data if there are certain problems.

Specifically: look for "NaNs", scream, and replace them with the Official NAOJ Invalid Value: 9998.0. Will handle the currently failing conversion elsewhere, but I think we will always be prone to this really catastrophic failure: NaNs are good at leaking.