Finish discussion on how many DITs in a FITS file

[hugobuddel]

From Instrument Data Description:

A template may produce several exposures, each of which consists of NDIT subexposures
(DITs). The exact definition of what constitutes an “exposure” or which and how many DITs
will be save in a FITS file, are still under discussion. The goal will be for a file to contain the
“smallest calibratable subset” of DITs, for instance one cycle of the chopping patters, or data
taken in one nod position. Keeping files small in this way allows the pipeline to process data
even for cases where observing blocks are only partially executed.

We need to discuss this and decide something.