Come up with a system for beamline specific zebra constants

[DominicOram]

Zebras on different beamlines tend to be wired differently. It is useful to refer to e.g. which output different hardware is on via a nicely named constant rather that the output number. In zebra.py we have some constants defined e.g. EIGER_TTL but these are only valid for i03. Ideally we would have these defined for other beamlines in a way that we can do:

def my_plan(zebra):
    yield from abs_set(zebra.in_1, DETECTOR_TTL)

and this works for any beamline. For example we could do this by when instantiating the zebra we also pass in the constants mapping.

Acceptance Criteria

• Nicely named Zebra constants are defined for all MX beamlines where we currently use bluesky
• It's clear which beamline, and which zebra, these constants are for
• We can easily write generic plans that will use these constants