Detector spectrum mapping: decide on mechanism and file formats #124

this ticket will involve:

• creating an ADR describing:
  • the problem statement - arbritrary format currently, opportunity to change, etc.
  • the file format we are going to use
  • the fact that we won't allow mapping to change mid-run
  • which producers/consumers will need to change
  • whether https://github.com/ISISComputingGroup/streaming-data-types/blob/master/schemas/df12_det_spec_map.fbs needs anything else tacked onto it
• creating a schema if needed
• creating follow-on tickets for actually implementing the change in the stack
• writing a note in the wiki with fundamental differences and incompatibilities with the ICP based system.
• creating a very basic script to convert files in the old format if needed.