Type for `ImagingContext.fieldOfView`

[vreuter]

Right now, fieldOfView is typed as FieldOfViewLike, which is equal to PositionName | FieldOfView. Either...
a) one of these types should be removed
b) ImagingContext should carry a type parameter like FOV <: PositionName | FieldOfView which would then be the type of the fieldOfView member, to disambiguate.

This has consequences for any type which uses ImagingContext, but it would make things like defining Order[ImagingContext] more reasonable; currently, this would need to be a partial function, since there's no real meaningful way to order a PositionName (text) value relative to a FieldOfView value (number).