Relative geometry expressions

[hamogu]

Currently, pos4d is always absolute to the coordinate system. It would sometimes be useful, if the position of an object could be set relative to some other position, e.g. a grating relative to the center of the support structure. That would simplify simulations where that support structure is misaligned. The GSA has several parameters to mimick this effect by recalculating absolute pos4d values with generate_facets but this could probably be simplified significantly if there was a mechanism to make linked pos4d values.

To do so, either self.pos4d could become a property that encapsulates that or this could be encoded in geometry('pos4d') and the direct use of pos4d would be removed.