reworked structure constructors to use mapcells instead of position vectors, did some random cleanup across atoms (trying to rename/rework the confusing instantiation code), added gravity and an artificial gravity effect on gravplates (little bit wonky, needs testing), tweaked mapsuite a bit

Signed-off-by: Miles Whiticker <miles.whiticker@gmail.com>
hardcoded limits to debug drawer rebuild quantities, some more optimisations

Signed-off-by: Miles Whiticker <miles.whiticker@gmail.com>