Use workspace start time when adding time-series log with AddSampleLog

[mantid-builder]

Original Reporter: Russell Taylor

This ticket is blocks : TRAC8637

AddSampleLog uses the current time (when the algorithm is run) as the time on the single-value TSP that can be added to a workspace with this algorithm. This seems like a very strange choice. A much more useful one (and one that's required for http://trac.mantidproject.org/mantid/ticket/8637) would be to use the workspace start time. '0' - the epoch start - would be better if that's missing (though it shouldn't be for real data).

[mantid-builder]

This issue was originally trac ticket 8638