Fix io bug with MAS model

[MohamedNedal]

In the recent version, there was a problem with importing MAS data files:

---> 14 mas_output = MASOutput(filename)
     15 rho = mas_output['rho']

File ~/.conda/envs/psp/lib/python3.9/site-packages/psipy/model/base.py:46, in ModelOutput.__init__(self, path)
     43 # Leave data empty for now, as we want to load on demand
     44 # This is a mapping from variable name to xr.DataSet
     45 self._data = {}
---> 46 self._variables = self.get_variables()
     47 self._variables.sort()

File ~/.conda/envs/psp/lib/python3.9/site-packages/psipy/model/mas.py:74, in MASOutput.get_variables(self)
     73 def get_variables(self):
---> 74     return get_mas_variables(self.path)

File ~/.conda/envs/psp/lib/python3.9/site-packages/psipy/io/mas.py:125, in get_mas_variables(path)
    123 var_names = [Path(f).stem.split(".")[0][:-3] for f in files]
    124 if not len(var_names):
--> 125     raise FileNotFoundError(f"No variable files found in {path}")
    126 # Use list(set()) to get unique values
    127 return list(set(var_names))

FileNotFoundError: No variable files found in rho002.hdf

I've modified the get_mas_variables function to be:

def get_mas_variables(path):
    """
    Return a list of variables present in a given directory.

    Parameters
    ----------
    path :
        Path to the folder containing the MAS data files.

    Returns
    -------
    var_names : list
        List of variable names present in the given directory.
    """
    path = Path(path)  # Convert path to a Path object
    files = glob.glob(str(path / "*[0-9][0-9][0-9].*"))
    # Get the variable name from the filename
    # Here we take the filename before .hdf, and remove the last three
    # characters which give the timestep
    var_names = [Path(f).stem.split(".")[0][:-3] for f in files]
    if not len(var_names):
        raise FileNotFoundError(f"No variable files found in {path}")
    # Use list(set()) to get unique values
    return list(set(var_names))