Reduce external configuration file

[dguff]

To run the simulation on the dune computing cluster, one should avoid using many external configuration files (e.g. for each material, geometry, ...) because the grid nodes cannot access the app area. To avoid a significant re-design of the code, we will reduce the external configuration files to just 2:

• the geometry configuration
• a single material database These two files will be passed as an input to the solar_sim executable

[dguff]

In b431c925e147045e9529c6639acda626a8701b88 we introduce more inputs for the executable (conveniently parsed with getopt). Among the new inputs, the user can specify the material database file (i.e., the json file containing the definitions of all materials) and the geometry description