Add feed flip term.

[JSKenyon]

Finally caving to @o-smirnov and @bennahugo's requests. This adds an explicit feed flip term. This can be included in your solver.terms by selecting type feed_flip. Note that if you require parallactic angle correction, you should use the newly added explicit parallactic_angle term and not the input_model.apply_p_jones and output.apply_p_jones_inv options. The current approach will up computational complexity - I will work on improving this but it will be a big job as I will need to refactor how the gains are collapsed within the chain. That will likely be a separate future PR.

Currently unanswered questions:

• Are the weights also flipped?
• Is simply including the flip in the chain sufficient to address the problem?