find_all_headers

    :param pipeline_files: files to process
    :param label_rules: labeling rules

def find_all_headers(
        pipeline_files=[],
        label_rules=None):
    """find_all_headers

    :param pipeline_files: files to process
    :param label_rules: labeling rules
    """

    log.info("find_all_headers - START")

    headers = ["src_file"]
    headers_dict = {"src_file": None}

    if label_rules:
        headers = ["src_file", "label_value", "label_name"]
        headers_dict = {"src_file": None,
                        "label_value": None,
                        "label_name": None}

    for c in pipeline_files:
        df = pd.read_csv(c)
        for h in df.columns.values:
            if h not in headers_dict:
                headers_dict[h] = "{}_{}".format(
                                    c,
                                    h)
                headers.append(h)
        # end for all headers in the file
    # end for all files to find common_headers

    log.info(("headers={}")
             .format(len(headers)))

    log.info("find_all_headers - END")
    return headers, headers_dict