Unify preprocessing site_pipeline and utils function into class objects.

[mmccrackan]

The scripts preprocess_tod and multilayer_preprocess_tod call a number of lower level functions from the site_pipeline util.py and the preprocess pipeline_util.py. This results in duplicated code and the recalculation of many parameters several times (such as the groups which calls context.obsfiledb.get_detsets(obs_id). This, along with things like error handling can probably be better handled inside of a class object structure that tracks the state of a preprocessing run at either the per obs-id or per wafer level. Many of the preprocess_util.py functions can be moved into a class and one can be instantiated for each process in a multiprocessing run.