Move aperture at the start of rotation

[olliesilvester]

Fixes #1336

Link to dodal PR (if required): https://github.com/DiamondLightSource/dodal/pull/573 (remember to update setup.cfg with the dodal commit tag if you need it for tests to pass!)

Not to be merged until the WIP GDA changes are done

To test:

1. Confirm changes satisfy issue
2. Confirm tests still pass

[olliesilvester]

After discussion with @DominicOram , this PR should now do the following:

• selected_aperture is an optional parameter of DiffractionExperiment

• XRC: If set to None, XRC won't change the aperture, and whatever is currently in position will be used. Otherwise, XRC will set the aperture according to the parameter

• Rotation: Same as above, and GDA should pass back the selected parameter from Hyperion's XRC

• Robot load: Inherits from DiffractionExperiment, but the plan always sets aperture to RobotLoad position, regardless of parameter. This is because, for now, robot load and XRC use the same set of parameters.

[olliesilvester]

Also, selected_aperture has a default of SMALL for gridscan, while for a general DiffractionExperiment it defaults to None