SolidGroupMerge objects to fix .obj imports

[PyMarc2]

• SolidGroupMerge is a Solid that takes multiple Solid as an argument and merges them in the same solid. It keeps the information about the solids by correctly managing the labels.
• SolidModel is a SolidGroupMerge but takes a filepath as an argument.
• added am example with the droid file correctly labelled.
• modified SurfaceCollection so it allows to add the same surfaceLabel, but increments it.
• allow solid.rotate(rotationCenter=Vector(x, y, z)) so we can rotate from another point then the center. Had to implement it when SolidGroupMerge had different objects in it, but it can be useful + it is tested, so I say we keep it.

[JLBegin]

PR title and description outdated since #71