Preparations to add tests of a bentonite material model to OGS

Soon, a rather complex material model for bentonite will be used in some ctests. This MR contains some prerequisites.

1. Some path modification logic is changed. That enables using a mesh directors (-m flag) together with relative paths in the project file. That change might not be strictly necessary, but might be handy from time to time.
2. one can now specify if an MFront behaviour <library> will be searched relative to the prj file. Setting this to false enables building behaviours for testing purposes only.

TODO

• add to changelog (behaviour change)

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?