Asteroid type names

Basically the same method as getAsteroidData(), but returns the name string instead of the CustomAsteroidData instance.

Verifying that the vessel is actually an asteroid can be done on either end, here or by the caller; I included here.

Similar methods could be used for the other CustomAsteroidData fields in case someone needs to know them before they are moved onto the ModuleAsteroid, but I don't have any need for them.