PYIODE: support Path objects for filepath argument everywhere

@yvdabb You may want to work on this. First:

• import Path from pathlib in the main file iode_python.pyx

Then, in every method/functions having a filename or a filepath argument in pyiode, you have to do:

• update the type hints of the argument filename/filepath (something like filename/filepath: Union[str, Path])
• update the accepted types for filename/filepath in the list of Parameters in the function/method documentation (filename/filepath: str or Path)
• in the body of the function/method, check that the type of the passed value for filename/filepath is either str or Path (isisinstance(filename/filepath, (str, Path))
• if filename/filepath is of type PATH (isinstance(filename/filepath, Path):), convert it to an absolute path as string (I think the method is 'resolve()') -> check documentation of pathlib
• check if the file exist -> if not file.exists(): raise ValueError(...)

See for example Variables.low_to_high() and Variables.high_to_low() methods