Implement all Dataloader subclasses as "plugins" (no built-in Dataloaders)

[lesslogic]

Certain data loaders are already hardcoded in PIVA. I suggest taking the current Dataloader subclasses out of data_loaders.py and implementing them on equal footing with user-made loaders (i.e., as plugins). Basically, make them "shipped" (as opposed to "built-in") features. Reasons:

• Stability & maintainability. Reduce risk to PIVA. Community can contribute/modify the loaders without touching core code. PIVA outsources all loader maintenance.
• Less mess for users. They can remove loaders they're not interested in. For the true globe-trotting facility users, the list of loaders can get really long. But also the majority of ARPES systems around the world are not in synchrotrons, so many or most users have no use for the built-in assortment of loaders.
• No implied bias toward or endorsement of specific institutes, facilities or instrument manufacturers.