Add default filename to qc2Data class

[Cmurilochem]

Add a default filename in which to save the formated qchem data. This would be extremely useful in situations where the users do no want or even forget to provide a specific filename. The updated code could be as simple as:

class qc2Data:
    """Main qc2 class.

    ....
    """
    def __init__(
            self,
            molecule: Atoms = Atoms(),
            filename: str = "qchem_data.hdf5",
            *,
            schema: str = "qcschema"
    ):