Install Dune modules from PyPI

Dune modules can now be installed via pip using the Python package index (PyPI). The module and the modules it depends on are downloaded, build, and installed either into the system or into a python virtual environment (the latter might be of more interest as it is an option to install Dune without root privileges). This could also be of interest for C++ users of Dune as Python bindings are not mandatory. Scripts like dunecontrol can be used the same way as with a system installation so that installed and source modules can easily be used together. More details on how to add this installation option to existing Dune modules is available here. Details will be added to the Documentation shortly.