Reads a GTF file and labels the respective columns in agreement with GTF file standards:
    'seqname','source','feature','start','end','score','strand','frame','attribute'.

    :param infile: path/to/file.gtf
    :returns: a Pandas dataframe of the respective GTF

def readGTF(infile):
    """
    Reads a GTF file and labels the respective columns in agreement with GTF file standards:
    'seqname','source','feature','start','end','score','strand','frame','attribute'.

    :param infile: path/to/file.gtf
    :returns: a Pandas dataframe of the respective GTF

    """
    df=pd.read_table(infile, sep='\t', comment="#", header=None, dtype=str)
    df.columns=['seqname','source','feature','start','end','score','strand','frame','attribute']
    #df = df.astype(str) # from DTC
    return df