Read a RES2DINV-style file produced by the ABEM export program.

def add_dat_file(filename, settings, container=None, **kwargs):
    """ Read a RES2DINV-style file produced by the ABEM export program.
    """
    # each type is read by a different function
    importers = {
        # general array type
        11: _read_general_type,
    }

    file_type, content = _read_file(filename)

    if file_type not in importers:
        raise Exception(
            'type of RES2DINV data file not recognized: {0}'.format(file_type)
        )

    header, data = importers[file_type](content, settings)

    timestep = settings.get('timestep', 0)

    # add timestep column
    data['timestep'] = timestep

    if container is None:
        container = ERT(data)
    else:
        container.data = pd.concat((container.data, data))

    return container