Add the shift applied to particles to the snapshot headers

Implement #852 (closed).

• Store the initial shift applied to all the particles in the space object.
• Write the shift to the output headers.
• Document this.

added feature request i/o labels

requested review from @yvesrevaz

assigned to @jborrow

Excellent ! Works for me !

from sw_getAttrs snap/snapshot_0010.hdf5 I get:

	 Shift
	 [30. 30. 30.]
	 size=3 type=float64

Thanks !

approved this merge request

merged

mentioned in commit bea26051