Documentation for hwp.py

[paganol]

We should write a minimal tutorial on the usage of get_pointings in case of HWP and some documentation for the class IdealHWP.

[paganol]

Vaguely related, should detector_quats in get_pointings be an optional argument with None as default? So that, by default, the code takes the quaternions stored in the observation. Again, should we have a wrapper function that takes a list of observations and produces a list of pointings? Instead of being forced to do something like this

pointings=[]
for obs in sim.observations:
    pointings.append(lbs.get_pointings(obs,
                      spin2ecliptic_quats=sim.spin2ecliptic_quats,
                      detector_quats = None,
                      bore2spin_quat=instr.bore2spin_quat,
                      hwp=lbs.hwp.IdealHWP(hwp_radpsec),
                     ))