misc BLD_ASTEROID_MINE changes

Hi, the attached patch to buildings.txt changes:
(1) PRO_ASTEROID_MINE_1 => PRO_ASTEROID_MINE
(2) effects = SetMaxMining Target.MaxMining + 8 instead of 4, to match text description
(3) stackinggroup = "ASTEROID_LOCAL_EFFECT"

I'm not sure about the last one, since there's no inherent reason not to allow multiple, but since the cost to build is currently 1, and maintenance cost doesn't matter, the player can just build an unbounded number of these to keep increasing mining output. It does make sense to have a more than 1 asteroid mining building (possibly as separate building types), but infinite is too much :)

Best,
Karl