Add option to include parallactic angle term in net gain

[landmanbester]

Describe the problem that the feature should address
The current apply_p_jones options only allow applying paralactic angle (or its inverse) on the fly. This is not compatible with the idea of applying gains on the fly during imaging.

Describe the solution you'd like
It should be possible to add the parallactic angle jones term to the net gain output. This way there is no ambiguity in the order in which terms are applied.

Describe alternatives you've considered
I have considered doing this inside the imager but I think it is cleaner to add it to the net gain because i) it avoids duplicating the calculation (and possibly getting it wrong somewhere) ii) there are fewer parallactic angle related options to confuse the user with and iii) there is less chance of messing up the order in which terms get applied. The latter becomes especially true when there are direction dependent gains involved.