Eliminate need for columns with lists as values (e.g. pathway functions)

[KelseyChetnik]

When a column contains lists as values (e.g. a column containing pathways such as those generated by mt_anno_pathways_hmdb) the function mt_write_se_xls does not work. Temporary solution, such as converting the lists to strings result in other functions no longer being able to use the column. This is also a test.