ScenarioManager needs a Read-Only mode

The ScenarioManager needs to check write permissions and switch to a read-only mode if it cannot write. This is important for installed packages and needs to be done before the next release (which we want to do next week), so would be great until Tuesday evening.

You can test this for some custom testpackage by calling

sudo make install
mv ../build build_old
export PATH=$PATH:$HOME/install```