Add Archive at a Specific Time

As part of the work for lhcb-dpa/analysis-productions/LbAPWeb#40 (closed) (and after dicussions with @cburr), this PR adds an extended version of archiveSamples called archiveSamplesAtSpecificTime that takes a datetime object as well as the list of ids and sets the validity_end field to this, thus allowing a sample to be archived in the future.

BEGINRELEASENOTES *AnalysisProductionsDB NEW: Added the function archiveSamplesAtSpecificTime that will set the end validity of the given samples to the given datetime. Essentially a minor extension to archiveSamples. ENDRELEASENOTES