Option to correct counts per sub-collimator

[grazwegian]

The current grid response calculates an average over all used sub-collimators as all counts are summed to form the spectrogram. For some use cases it would be more useful to reproduce the modified counts for each sub-collimator and include just the average value in the response.

[ennosigaeus]

We discussed this during the STIX co-location. This was requested by Karol. Useful, but maybe lower priority. It could be implemented in Python instead of IDL.