Expose gravity information at runtime

[PFQNiet]

I'm trying to make a "flight time calculator" tool, and that is a function of:

- Thrust
- Platform weight
- Platform width
- Distance
- Gravity at origin and target

Thrust can be tricky since that's adjustable with throttling but I'll worry about that later. The big thing missing is the gravity values, as these are only defined on the prototype stage and not exposed on LuaSpaceLocationPrototype.

I would be very grateful to have this information, because when I hardcode it for testing purposes I get flawless results.

[braxbro]

For now as a workaround you can rip those values in final-fixes and put it in a ModData object, but I do agree - no reason not to have that exposed as a prototype member in runtime.

[protocol_1903]

+1