docs: Minor corrections, fixes to ControlPort documentation.
docs: Updated and improved the message passing docs page.
docs: added information about the new pfb_arb_resampler kernel.
docs: added documentation of the stream tags interface.
docs: added doc page on Performance Counters.
Merge branch 'maint'
grc: added special variable objects to represent constellations.

"Constellation Object": represents the 'calcdist' version, which is generic but uses Euclidean distance to make decisions.

"Constellation Rect. Object": special rectangular constellation with a simpler decision calculation.

digital: also updated documentation for constellation classes.
controlport: Added probes for byte, short, and int data types
(ctrlport_probe2_x).

Had to add some more plumbing to ControlPort to handle different data
types to support the new probes.

TODO: in 3.8, we will remove ctrlport_probe_c and make a single GRC
file for all data types and remove blocks_ctrlport_probe2_c.xml.
blocks: Added QA code for all ControlPort probe types.