Lacking access to convert_units when creating grids

[Jakub W. Both]

At the moment, when creating grids, under the use of customized units (via pp.Units), it is not possible to use the convenient functionality self.solid.convert_units(<val>, "m"). See as example the tutorial single_phase_flow.ipynb. In order to fix this, the method set_materials() has to be called before set_geometry() within SolutionStrategy.prepare_simulation(). After the change, the tutorials and other places in the core should be also updated to suggest consistent use of convert_units.