Tidy up and document constants/feature flags

There are a number of constants/feature flags that are beamline scientist facing. e.g. grid scan exposure or grid scan transmission.

Some of these are dotted amongst the code, some are in GDA properties etc.

We should tidy these up so that any unused ones are removed and that what's left is as clear as possible. We should then document what is where so that a scientist is able to modify it themselves.

## Acceptance Criteria
- Constants are tidied up
- We give the docs for this to Neil/Mark/David and ask them to change some parameters and they should be able to w/o any help from developers