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:

 - `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).

See merge request !145