Add functions for export, run, read output of wrapper `run_simulation()`

Currently, run_simulation() takes an OGS6 instance and exports the ogs6 input files, executes the simulation and retrieves the simulation output. I think the functionality should be splitted in a function that exports the input files (this is already there, please just export it), a function that runs the simulation and a function that reads the simulation output. You can still have a wrapper such as run_simulation(). Yet sometimes, its necessary to have accessible functions for these individual steps.