grid_width_microns is required by grid_detection_plan but is not in the experiment params schema

[rtuck99]

Currently grid_width_microns is required by grid_detection_plan. It is obtained from the GridScanWithEdgeDetectParams.grid_width_microns via PinCentreThenXrayCentreInternalParameters. However in wait_for_robot_load_then_centre_plan the upstream PinCentreThenXrayCentreInternalParameters is constructed from WaitForRobotLoadThenCentreInternalParameters which doesn't have this attribute, and it is also absent from the corresponding json schema.

This may possibly be mitigated by GDA populating this field anyway, but really we should not be round-tripping parameters via json between unrelated types as this is a source of potential bugs

[DominicOram]

If this isn't breaking anything and is just a refactor the idea is to do the bigger parameter refactor instead