Returns a field of choice from the attribute column of the GTF

    :param field: field to be retrieved
    :returns: a Pandas dataframe with one columns containing the field of choice

def retrieve_GTF_field(field,gtf):
    """
    Returns a field of choice from the attribute column of the GTF

    :param field: field to be retrieved
    :returns: a Pandas dataframe with one columns containing the field of choice

    """
    inGTF=gtf.copy()
    def splits(x):
        l=x.split(";")
        l=[ s.split(" ") for s in l]
        res=np.nan
        for s in l:
            if field in s:
                if '"' in s[-1]:
                    res=s[-1][1:-1]
                else:
                    res=s[-1]
        return res

    inGTF[field]=inGTF['attribute'].apply(lambda x: splits(x))
    return inGTF[[field]]