Allow multiple parts to be represented within a single voxel / other ways of handling shielding for overlapping parts

[ferram4]

See single-thruster RCS in RO being shielded when multiple parts are placed in proximity to each other on a large enough vehicle, where the voxel resolution is on the scale of the part. Both parts fit within a single voxel and the single part per voxel + special override for functional modules assumption fails, causing one part to be shielded and inoperable and the other not.

Look into representing multiple parts within a voxel (risk of much higher mem usage due to array / list being required) or possible alternate ways of identifying when this particular error occurs.