Need a generic spectrograph class

[jhennawi]

Hi All, I think we should have the option to instantiate a generic spectrograph
currently this is not possible for cases where 'INSTRUME' didn't get written to the header.
I'd like to be able to access things like standard header cards or whatever
I think the load_spectrograph(None) should return this generic spectrograph instead of None
Right now if I try:

from pypeit.spectrographs.spectrograph import Spectrograph
spectrograph = Spectrograph()
returns errors because it cannot validate the meta data model
Thoughts?