Adding unique_pair keyword to obs_nodding_irregular for Issue #11.

[Hoeijmakers]

This is a temporary fix for Issue #11 to deal with regular nodding sequences containing multiple exposures per nod.
obs_nodding_irregular has received a new keyword unique_pairs, which if set, will enforce a unique pairing between each A and B nod in a linear order.

Consider the following AAAABBBB BBBBAAAA sequence of numbered A and B frames:

1A, 2A, 3A, 4A, 5B, 6B, 7B, 8B, 9B, 10B, 11B, 12B, 13A, 14A... etc.

With unique_pairs=True, the following nodding pairs will be passed to cr2res_obs_nodding:

1A and 5B
2A and 6B
3A and 7B
4A and 8B
5B and 1A
6B and 2A
7B and 3A
8B and 4A

9B and 13A
10B and 14A
etc.

This is not the perfect solution for dealing with multiple exposures per nod (as discussed in Issue #11), but will work as a temporary fix and should generally be OK for time series where the background is not high compared to the target trace (K band and shorter). Also note that this invalidates half of the cr2res_obs_nodding_combined frames, and only the cr2res_obs_nodding_extracted frames, or at most only the A or B variant of the cr2res_obs_nodding_combined frames should be used.

[tomasstolker]

Thanks @Hoeijmakers! I did not test but from what I can tell this looks all good 👍