BUG: 0002831 - allow loading of secondary capture dicom files

Sometimes people create 'secondary capture' (SC) dicom types as a byproduct
of anonymization or other data processing. While is really meant
to be a 2D screen capture, when you have a stack of them it is
sometime desirable to load them as a volume. So with this change
we allow loading with a warning, and with a low confidence so that
some other plugin can precome the default selection if it is better
able to load this data.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21780 3bd1e089-480b-0410-8dfb-8563597acbee