Multi-tile Units does not handle damage correctly

[perara]

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https://github.com/cair/deep-rts/blob/148df8d8cdd628697b5f830ed28b1774e9f2c4cf/src/Tile.cpp#L262-L270 In multi-tile units, the damage is only applied to the "primary tile". Units currently only reference1 tile, but perhaps there should be a list of "secondary tiles"