Standardise VMXm FGS IOC

VMXm's FGS IOC currently doesn't include a PV which i03 and i04 has. This boolean PV should look at the FGS parameters and validate them against the sample stage motor limits. It isn't as crucial for VMXm to have this check, since the grid scan will still be triggered through GDA, which has its own checks, but it's still I useful thing to have.

We should also:
- Get a PV added to show what motion program is currently running.
- Standardise dwell_time/exposure_time (they are used in the same way)
- Standardise the POS_COUNTER PV by having separate `POS_COUNTER` and `POS_COUNTER_WRITE` PV's

I have asked controls to add the first two PV's already

## Acceptance Criteria
- PVs added, tested and deployed