[cmake][python] use `pip --upgrade` on install

This patch will always upgrade the Python package upon make install.

Previously, the Python package was only installed, if the package had not been installed before. This violates the principle of least surprise as we intuitively except make install to overwrite all files during the install procedure.

This fixes #81 (closed).

@andreas.dedner, @dominic: Would you care to have a look?

@nolte @dominic: fine with me

Unless there are objections, I will merge this one tomorrow.

mentioned in commit bcafed0b

merged

mentioned in commit 8e4418c6

mentioned in merge request !426 (merged)