Tidy up MJPG and SnapshotWithGrid structure

Currently `SnapshotWithGrid` inherits from `MJPG`, both of these are expected to be used in an OAV device. This is quite clean as one provides the snapshots with the grid and one without. However, because of the way Ophyd v.1 creates devices it means that it's not easy to choose at runtime which should be used without:
* Having the OAV contain both devices
* Having two OAV objects, one with grids and one without

We should decide if there is a better way of doing this (do it in v2?) or which of the two approaches above we prefer