Convert dataframe into list of vertices, assuming that vertex ids are stored in _VID_COLUMN.

def _dataframe_to_vertex_list(df):
    """
    Convert dataframe into list of vertices, assuming that vertex ids are stored in _VID_COLUMN.
    """
    cols = df.columns
    if len(cols):
        assert _VID_COLUMN in cols, "Vertex DataFrame must contain column %s" % _VID_COLUMN
        df = df[cols].T
        ret = [Vertex(None, _series=df[col]) for col in df]
        return ret
    else:
        return []