Abstraction for the `MerlinControl` interface

[sk1p]

• Commands need to be ordered -> maybe batch them into transactions?
• Provide pre-build common trigger setups as classes, while still allowing for customization (example: pixel/line/"complete dataset" trigger using different triggers (edge/lvds/internal where it makes sense))
• Make it as easy as possible to customize connections to microscopes/scan engines
  • include the CEOS client library in LiberTEM-live?
  • needs to mesh well with the new merlin control abstraction