Add methods for listing registered classes/devices from python interface

This MR aims to expose the listX methods that exist under the various registries (e.g. listPowerSupply). This will enable one to do:

from labRemote import ps
...
ps.listPowerSupply()
# [RigolFoo, AgilentBar...]

This will make it easier for utilities to inspect the known devices/etc. So far I have the methods for the power supply register, com register, and datasink registry. I think this is all of them at the moment.

Tagging @gstark