Added support for rtl_tcp

[hennichodernich]

I added support for rtl_tcp osmosdr source blocks because I needed it for a virtualized receiver. Maybe it is of value for others, too.

[igorauad]

Hi @hennichodernich ,

Thanks for the changes. They look good to me, aside from the formatting. I will accept and fix the formatting on a subsequent commit.

I use yapf and flake8 to format and lint Python, respectively, but the CI was not checking those. I will fix that too.

Thanks