Add ways to dynamically mirror structures of entities in `bs.link`

[VForiel]

Suggested by @theogiraudet

This module will essentially rely on the bs.link module. It will encapsulate the offsets and hide conversion between axis (on x, y and z) rotation and natural Minecraft rotation (phi and theta)

This function will contain first these two functions:

• bs.mecanic:define_pivot_ata which will link an entity to a point in space instead of linking it to an entity
• bs.mecanic:rotate_[x,y,z] which allow to rotate the structure from the pivot point