o.c.archive: Fix linear interpolation cast error f. non-numeric samples

.. as can happen when exporting a time range from the data browser that
includes "Disconnected" samples
Merge remote-tracking branch 'origin/master' into
bug228_archive_export_linear_interpol

Conflicts:
	applications/plugins/org.csstudio.archive.rdb/ChangeLog.html
	applications/plugins/org.csstudio.archive.rdb/META-INF/MANIFEST.MF
Merge pull request #232 from ControlSystemStudio/bug228_archive_export_linear_interpol

o.c.archive: Fix linear interpolation cast error f. non-numeric samples: Fixes #228