docs: fixed doxygen warning.
docs: fixed exploring-gnuradio examples and added to doxygen manual.
pmt: added class comparator to rename comperator (to be removed in 3.8).
blocks: prevent alowing more input streams than this block uses.
docs: fixed/modifying/adding documentation for blocks a-l.
digital: fixed warnings about order of initializing variables in constellation.
Merge branch 'maint'
Merge branch 'master' into next