Acquisition viewing and procedures should produce acquisition objects

[cmeyer]

Currently data acquisition, such as scans, can produce multiple data items simultaneously. Computations should have some mechanism to ensure that they are operating on a consistent set of data items. For instance, if one were to subtract one scan channel from another, it makes sense that the computation should not be acting on one images acquired at t=0 and another at t=1.

See also:

• https://github.com/nion-software/nionswift/issues/942
• https://github.com/nion-software/nionswift/issues/663

  
  ### Tasks

• [ ] https://github.com/nion-software/nionswift/issues/434

[cmeyer]

erDiagram
  Acquisition||--|{AcquisitionItem:reference_connection
  AcquisitionItem||--||DataItem:reference_connection
  Acquisition {
    id uuid
    index int
    name string
    procedure_description dict
    instrument_state dict
  }
  AcquisitionItem {
    id uuid
    channel string
  }