Support non-square 2D grids

[w-m]

For simplification, the current implementation only supports square grids. For broader use, it would be useful to extend support to rectangular grids.

[w-m]

It doesn't crash when sorting non-square grids, but the code in the grid reorganization has only one variable for block size and number of blocks (independent of x/y dim), e.g. https://github.com/fraunhoferhhi/PLAS/blob/main/plas.py#L238-L239 - thus it doesn't properly sort the non-square overhanging part.

Doesn't really make sense fixing this state of the code for non-square grids, as it contains the overly complicated but dead/unused pixel unshuffling.

-> do non-square support after rewrite