[BUG] Inconsistencies in the name of the SUBCELL_RSD option

[zhaotingchen]

Describe the bug:
Not really a bug, but in the python interface, the parameter for turning on the is (p21c.FlagOptions._defaults_)['SUBCELL_RSD']. But in the documentation, the parameter is named as SUBCELL_RSDS with a trailing ``S''.

Expected behavior:
Consisntencies between the documentation and the code.

Details:

• 21cmFAST version 3.3.1

I want to also use this opportunity to ask if there is any other parameter to tune for getting the RSD effects. In particular, I am not sure if T_USE_VELOCITIES in GlobalParams should be turned on. It seems to me that SUBCELL_RSD is a more accurate version, but I am not too sure.

[steven-murray]

Thanks @zhaotingchen. We can update the documentation for this. I don't think there's anything you should need to tune for RSDs. You can choose the axis of the box that is used as the LoS direction, but there's no real point in changing it.