Draft: Restructure feflowlib

Aim is a full integration to the meshclass. A goal for the API is defined in #62. Further (old) planning can be found in the wiki.

Furhter ToDos:

• rewrite cli
• think of write_mesh_of_combined_properties
• rewrite examples (delete links)
• think of new function for boundary conditions (maybe also function for writing) or introduce BC-class
• introduce meshlib.Mesh class as type for ogs_specific meshes?
• introduce consistent SI-Unit conversion for all possible parameter
• think of renaming parameter in feflow-language to ogs-language
• refactor templates, so no code duplications occur
• allow time steps, initial time and end time to be arguments for all processes in templates
• fix permeability for HT process
• introduce generic template for processes, not defined
• allow SSD as process in feflowModel
• rename feflowModel->FeflowModel
• bug fixes Hailong
• remove -i -o from cli and use same path as input, if no output is specified
• edit log, so only with --verbose you get detailed log. maybe create a log-output file.
• adjust test for large model
• if MaterialIDs in user-data overwrite them in bulk mesh