box_size_um is required by GenericGrid even though x + y box dimensions can be specified independently

Vmxm only run the gridscan and do not use the grid detect, however still have to populate `box_size_um` despite it not being used in gridscans.

We should probably separate out the grid specifications for grid detect and grid scan and not require the unneeded fields to be populated especially if the different axes could potentially be specified differently.

## Acceptance Criteria
- Gridscans don't need to specify box_size_um or alternatively,
- Vmxm gridscans specify only box_size_um and compute x/y dimensions from this