Handling of multiple Projections

[dylanmcreynolds]

Currently, the the code in databroker.projector has helpers to select a single projection from a list of possible projections identified in the start document, either by name or, if name is not provided, but selecting the only projection. If not found, it returns none.

@ronpandolfi argues for an option to project all projections provided in the start document. So, the return of a projector call would be a tuple of all datasets projected by the the various projections. I think this is a nice idea, but should be considered carefully. Perhaps:

If  a projection name is provide, then only use it
Else project them all!