FIX: get_output_filename() of node copy
Test user-defined selections for bss_regr node
FIX: tests for bss_regr
EHN: More flexible feature definition for ecg_annotate

Node ecg_annotate is now able to extract an arbitrary number
of features from the events marking each block of interest.
Before this commit node ecg_annotate marked each block with
the corresponding event selector name only.