x_old is now a x_diff since the last tree build. It is also implemented for the gparts.

Two changes here following the discussion in #122 (closed):

• x_old is replaced by x_diff, i.e. the offset since the last tree-build. That makes the drift task a bit strange as they have to move x in one direction and x_diff in the other.
• The gpart structure also gains an x_old field. This is necessary for the cases where we only have gparts and no parts. If we don't do this, the tree reconstruction is never triggered in engine_marktask().

A neat consequence is that we can store x_diff as float and not as double. So the xpart structure goes down to 32bytes in size (after alignment).

Reassigned to @pdraper

Good stuff! Re-assigning to Peter so that he can go ahead and merge this.

Ok, so this one should be good to go.

Status changed to merged

mentioned in commit ab93450e

Passes simple tests, so accepting.