Changing the conversion formula for a unit doesn't invalidate the snapshot cache

[Nicklas Nordborg]

The snapshot cache is used to increase performance when loading annotation values. The values in the cache should be the same as the values in the database.

If the conversion factor for a unit is changed all annotation values that are relative to that unit are re-calculated, but the snapshot for the affected annotations are not invalidated.

Fortunately, conversion factors between units never change in the real word, so the only way this can happen is if a mistake has been made when registering new units.

[Nicklas Nordborg]

(In [6532]) Fixes #1845: Changing the conversion formula for a unit doesn't invalidate the snapshot cache