Fix photoionization correction to conserve cS2

[zihaoxu98]

What does the code in this PR do / what does it improve?

This PR fixes the implementation of photoionization correction. Since the time dependence of total S2 has already been corrected by SEG correction, in photoionization correction we should ensure total S2 is not changed but only AFT is corrected.

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• [x] Update the docstring(s)
• [x] Update the documentation
• [x] Tests to check the (new) code is working as desired.
• [x] Does it solve one of the open issues on github?

[coveralls]

coverage: 91.795% (+0.03%) from 91.77% when pulling 76833267e1fc1e1208c5e90c8e66a64771014caa on pi_cor_fix into bad445fec640e9208bc40f6fde368df1ca9babc6 on master.