Investigate using stricter units on signals

[DominicOram]

As an MX DAQ developer we often have issues where we get confused about what units physical quantities are being expressed in (e.g. DiamondLightSource/mx-bluesky#220). It may be useful if we could be more strict about this in the code by leveraging something like pint. This would mean signals are set with Quantities that have units and will throw exceptions if the units given in the EGU field of the PV do not match. This is a spike to do the investigation on how hard this would be and what an interface would look like.

Acceptance Criteria

• We investigate using a library like pint to allow setting signals with values that have unit metadata
• We investigate how much this would change bluesky etc.
• An estimate for the work is written up, with a mock up of roughly what the interface would look like on the plan/device side
• It is decided whether we do this or not and an ADR is made