Add fields to start document to databroker for xpcs

[dylanmcreynolds]

No description provided.

[dylanmcreynolds]

This task has two parts. Group will define extra fields available to use in start documents. These fields will be approximately:
{"experiment":
[
["tomography", {"mode": "scanning"},
{"projections": ["primary", "fastccd"], "sinograms": ["sinograms", "data"], "angles": ["primary", "theta"]}
],
["scattering", {"mode": "transmission"}, {"scattering_detector" : ["primary", "fastccd"]}],
["scattering", {"mode": "reflection"}]
]
}

Once the beamline's databroker instance is configured to provide these fields, the xi-cam plugin can be coded to read the fields to know how to treat the data.