Update PMT plot for nT

Created by: JelleAalbers

This renames straxen.plot_on_pmt_arrays (#44) to the more convenient straxen.plot_pmts, and lets it use the nT PMT positions by default. You can still get xenon1t by passing xenon1t=True. I also moved things around in common.py a bit, to group all the old 1T stuff together.

Example:

import straxen
straxen.plot_pmts(np.arange(494), label='PMT number')

You can also pass show_tpc=False to get: