Draft: swappable objectives for upright configuration

This MR will enable the objective to be repeatably swapped on the upright - each objective sits in a triangular plate with three magnetic balls (thanks UC2 for that idea), and can slide in to a kelvin mount that attaches to the top of the optics module. Currently the Kelvin mount works (just about) and the optics module should be OK, but it fouls the Z mechanism and will need the mounting screw to be moved, and a taller spacer.