Simpler projector

[dylanmcreynolds]

Currently, databroker.projector:project_xarray provides an xarray.Dataset instance with projected fields renamed according to the selected projection. One issue with this design the return requires a fair amount of prior knowledge about how where in the return xarray structure to find the "stuff" from the run. Currently:

start document metadata fields can be found in the return_xarr

• ay.attrs
• stream event data is returned in the return_xarray.data with field names from the projection
• stream event configuration fields are returned in the stream's xarray

.attrs under the "configuraiton" key.

In a call with @danielballan , we talked about potentially adding a second projector (or potentially replacing this projector) with something that's a more custom datastructure with more inherint meaning to the caller.