3254 feature bec epoch

I added functionality to our bec plotting that lets you plot against the unix epoch.

To enable add the following to the metadata of a plan.

'legend_keys' : ["det1", "det2", "det3"],
'use_unix_epoch': True,

Adding 'use_unix_epoch' will use the unix epoch rather than the start time to plot against when there is no motor in the plan, e.g. in count

Adding 'legend_keys' allows you to change the strings used in the legend

I was unable to write a test for this since it involves the generating of a plot. Suggestions on how to write a test are welcome!