neutral_f_pn flag default behaviour

[mikekryjak]

If not solving neutral momentum if the neutral_f_pn flag is set to True, SD1D will overwrite the plasma momentum sink term F with the gradient of the neutral pressure, likely to maintain a force balance in the case. It is set to True by default.

This feature can be useful because it ensures that default behaviour leads to momentum conservation. However, this behaviour is not described in the manual, the name of the flag is not clear and the individual force sinks are still calculated and outputted (Fiz, Frec, Fcx, Fel) even though they will not be used for F, which can be misleading.

I suggest:

• Renaming the flag to force_f_balance
• Including a description of its behaviour in the manual
• Setting Fiz, Frec, Fcx and Fel to 0 if the flag is set to True
• Maintain default behaviour as True