Add option to overwrite directory when saving results

[livoire13]

The current version of femtoscope does not allow to save simulation results using ‎LinearSolver.save_results(self, name) if a directory with the same name already exists. This results in a FileExistsError and the data may get lost. To avoid this, use e.g. mkdir(exist_ok=True) (pathlib method) together with a custom warning, or use a try/except block. Another option is to rename the result to be saved or the already existing directory.