Restructure code base and ensure tests still pass

[rosesyrett]

No description provided.

[coretl]

Pretty much there, I'm more on the "multiple classes per file" side of the fence, but a quick poll of the office suggests I'm in the minority, so I think you are right with your file splitting decisions.

I would like to reduce the import namespace however, so the top level namespace should be:

• core
• epics.signal
• epics.areadetector
• panda
  and those namespaces should be populated with __init__.pys that only import from modules that start with _