Add parameter (and subsequent loading in) of PCAL Foton File

Need to be able to import "S" or "susnorm" and "mpN_DC" -- the suspension transfer function normalized by the "whitening" filter (the 2 poles at 1 Hz), in the event that it has systematic error.

So, I guess we'll need two new variables in the .ini file that defines the foton text file location and filter bank EPICs gain, and then a new function module in the pcal.py class that reads in and contructs everything in the (IFO):CAL-PCAL(END)_(RX,TX)_PD filter banks.

(This function module will also be useful for computing the PCAL force from the channels that are which is stored in the frames, rather than "just" the displacement).