modtool: fix finding gnuradio-runtime

pkg-config does not set variables when falls to default values, particulary include dirs,
thus we need check only if libs are set
modtool: fix detection of GNU Radio module directory
removed *out from sinks, *in from sources
examples: Some of the examples were updated to use 3.7 API, but the new module namespaces were never imported.
Merge branch 'maint'
Merge remote-tracking branch 'marcusmueller/improve_source_sink_c++_template'
docs: Fixes sphinx docs generating script to use any current LD_LIBRARY_PATH already set by the user.
Merge branch 'maint'
digital: fix digital.cpmmod_bc XML for new constants location
Merge branch 'maint'
runtime: fixed copy/paste error in volk_types.py
Merge branch 'maint'
added tag_propagation_policy_t to block_gateway
channels: Added the hardware impairments modeling tools from Matt Ettus' GRCon11 talk.

Python blocks part of the gr.channel module.

Examples are in gr-channels/examples (demo_gmsk, demo_qam, demo_ofdm, and others).
qtgui: Added histogram sink.
channels: using new qtgui histogram sink in quantization demo.
docs: added more documentation to the channel models page explaining the diffenerent models available.

Includes discussion of the new HW impairments model that is Pytho-only.
qtgui: improved handling of scale in histogram plots.

Mouse scroll zooms in/out horizonatally. Added auto-scale feature, can set x-min and max in constructor.

Mouse scroll in constellation plot now does both axes at the same time.
channels: updated quantization demo for new histogram constructor.
digital: ofdm benchmark_tx/rx added lo_offset option and enabled external clock utilisation