Adding Radx to the VM.

I added the installation of Radx to the Vagrant scripts via `provision_scripts/install_radx.sh`. I also added the Sphinx documentation accordingly so it reflects that the VM contains Radx.

In addition, I prepared kind of a `radx_short_course` template that is based on the IPython notebook, similar to the courses we designed for the other packages. See https://github.com/heistermann/radx_short_course. The idea is to execute Radx applications from IPython using the "!" operator and then read and visualize the output e.g. by Py-ART or wradlib. As I said, that repository is more like a template right now, without much meat. However, it is already installed on the VM, too, via  `provision_scripts/install_radx_short_course.sh`, and contains a NEXRAD sample data set.

Finally, the PR also contains a small hotfix for wradlib: wradlib needs a couple of additional dependencies lately, so I added these to the  `provision_scripts/install_wradlib.sh`.

Building the VM using vagrant worked on my machine, but someone else might want to test it, too. Once we approved the PR, we need to upload the new VM images to SourceForge.