A way for vessels to cache "accumulatable" values

[Katniss218]

Since I'm bad at writing...

Example: mass
When mass is defined, it's spread out all over the vessel, in IHasMass components.
The vessel itself should know its total mass and center of mass (and moment of inertia) at any given time, but it's expensive to recalculate everything.
Thus it already supports caching of the mass, and updating its value whenever a mass of the component changes.

I want to extend this to other cacheable values, in a way that mods will be able to define their own too.