raise an error if new files are found in project folder before reset()

[Gautzilla]

Calling project.reset() on a project that contains additionnal files/folders in its root folder will now raise an error:

RuntimeError: The following files would be deleted by a reset.
Either delete them manually or move them to the C:\Users\berthoga\Documents\Datasets\example_dataset\other folder before resetting the project:
yo.txt

resolves #379

[coveralls]

coverage: 98.878% (+0.005%) from 98.873% — Gautzilla:reset-with-added-files-raise into Project-OSmOSE:main