Add 'dims' to data_key

[danielballan]

Proposal

Add an optional dims property to the definition of data_key in Event Descriptors. The value of dims should be an array where each item is a string. For a very relevant example of how to specify this in jsonschema, see shape in the linked section of code.

Motivation

Numpy arrays have a shape with N entries corresponding to ndim the number of dimensions in the array. Event Descriptors have a corresponding shape property, declaring the shape of the data in some field of an Event.

Xarrays add string labels to the array axes, called dims ("dimensions"). It would be good if ophyd Devices that return array day could suggest names for the dimensions, like dims=('exposure', 'y', 'x') for AreaDetector. It would would natural to put this in the data_key next to shape.

Then databroker can use this information to label the dims in an xarray correspondingly and make the data easier for the scientist to interpret.

[danielballan]

Closed by #27