Update GNU Radio flowcharts to version 3.8

See merge request !1

The former version did not correct the bit errors. Bit correction
was only done in a local variable which was then thrown out.

This should fix several problems with the build system.

Python 2 has to be used explicitly or else cmake will pick the
libraries from python 3 in my system and swig won't work, preventing
one to import tnc_nx into python2.