Update processing level implementation to new definitions

[leewujung]

In April 2024 we updated the processing level definitions in https://echolevels.readthedocs.io/en/latest/levels_proposed.html via discussions between @leewujung @valentina-s @brandynlucca .

Let's update the implementation to reflect these changes.

The main change is that we switch to using sublevels A and B to denote data products before and after some kind of QC steps within each level.