Add `Calculate` -> `Midpoint Between` / `Centroid Between` to context menu

[adamkewley]

Specific use-case, but some parts of building a model require doing things like placing stations/markers on bodies and then calculating the centroid between two of them (e.g. the ISB definition for a knee joint works this way). It'd make it easier to figure out where the midpoint is if there was an appropriate Calculate option for it.