hwp doc and change in get_pointing

[paganol]

This PR addresses the issues described in #197.
I made the following changes:

• In get_pointings, the parameter detector_quats became optional. Now, not passing this parameter, the function takes detector quaternions from the observation passed.
• A new function was created, called get_pointings_for_observations, which wraps get_pointings. Now you can pass a list of observations to this function and a list of pointings is returned. This function provides a nice shortcut with a simplified interface with fewer options. Of course, for a more refined simulation, get_pointings must be used. The logic is the same we used in add_noise_to_observations, scan_map_in_observations, etc...
• Relevant tests were also fixed and they all pass on my laptop. On GitHub, they fail, but it seems to be a problem not related to my commits.
• The documentation is fixed accordingly. I also added a small extra entry in the section Scanning strategy for describing how to use the HWP class with get_pointings.

[ziotom78]

@paganol , sorry for the mess, I mistakenly merged #160 here and had to revert it. Now the CI builds pass, this PR can be merged.