Moved OVC initialization steps to methods

[colleenjg]

Moved initialization steps in ObjectVectorCells to separate methods, to allow these to be overridden more easily by child classes.

This can be useful for creating an ObjectVectorCells child class where the assignments of objects to cells is not uniformly random. Like this, a minimalist child class can be written where just a method or two are overridden, but the original init can be kept.

[TomGeorge1234]

Thanks. Stuff like this should be done more broadly tbh, an upgrade (as you pointed out) for v2.