Timed outputs

This implements the improvement #142 (closed).

The time of the first output and the time difference between subsequent outputs can be specified in the parameter file. So no dumps after a fix number of steps any more.

Users can also specify a unit system for the snapshots. That gets passed along to the routines dumping the snapshot.

I have updated all the YAML run scripts as well.

mentioned in merge request !160 (merged)

Works with the various levels of HDF5 support and different timings, so accepting.

OK to remove source branch?

mentioned in commit 168f0bd9

Status changed to merged

mentioned in issue #161 (closed)

mentioned in commit e5a9a253