Adding C++ Code generation support for c++ implemented blocks

[Martin Hübner]

Dear GNU Radio Community,

for a university project we want to use GNU Radio / gr-satellites to 
decode some signals of S-NET satellites. The goal for this semester will 
be, to provide a software that enables easy communication with them for 
HAMs, either by using traditional transceiver or SDR.

As the software will potentially be used by a broad group of differently 
experienced persons, we want to keep the deployment as easy as possible: 
A single statically linked binary would be ideal.

We already managed to setup a receiving pipeline in GRC like shown by 
DK3WN and EA4GPZ here:
https://www.dk3wn.info/wp/satelliten/s-net/ (GRC-Screenshot quite 
below). Next, we want to try to invert the flowgraph, to tackle signal 
generation and later on transmission too.

Unfortunately there are 4 blocks in the flowgraph, that don't support 
C++ code generation:

- Frequency Xlating FIR Filter
- Quadrature Demod
- Sync and create PDU
- S-NET deframer

For the first two blocks, it looks like they are implemented completely 
in C++. So I hoped adding support for them would be rather easy. I 
oriented for that on Håkon Vågsether work done here:
https://github.com/haakov/gnuradio/commit/18cbe62c27c3dc8006b707a172da80300e2adfa8

Similarily, the C++ support for the rather easy quadrature demod block 
should look like this then, right?
https://github.com/Akira25/gnuradio/tree/add\_cpp\_generation-quadmod

We would love to upstream these changes too, if you find them useful.

Kind regards,

Martin