Get instrument mode for TA data

[bhilbert4]

For most observations, the exp_type and the instrument mode are synonymous. But for some TA observations, the instrument mode needs to be retrieved from a header keyword and fits extension that are dependent upon the type of TA observation. @penaguerrero knows how to do this.

Should we add this to the Django models as the exp_type for TA data? Or do we want to keep the exp_type as e.g. NRS_MSATA, NRC_TA, and then have a new entry for the instrument mode? Woudl that be wasteful if mode and exp_type are identical 90% of the time?

This information would be used in the hover over improvements described in #1019
@BradleySappington