Dfilter constants

added:

• FILTER_ALL, to be passed to dfilter only and result in filtering/masking all flagged data
• FILTER_NONE, to be passed to dfilter only and result in no filtering/masking at all

closes #261 (closed)

assigned to @palmb

changed milestone to %2.0

added accepted category: (global) constants category: public API feature labels

requested review from @schaefed

changed the description

approved this merge request

merged

mentioned in commit aa3544b1