Diffuse sky radio noise with GSM

Clone this repo & compile gsm + skymap with the provided Makefile as:
```
git clone https://github.com/TREND50/skymap
cd skymap
make
```
Tabulate the radio signal as frequency using GSM. See run-gsm.py.
Compute the corresponding antenna noise using the provided effectie area and skymap. See e.g. compute-skymap.py.

The results can be checked with the show-gsm.py and show-skymap.py scripts.