rank aggregation method, by defaylt 'RRA', other options are
'min', 'geom.mean', 'mean', 'median' and 'stuart'
Value
a dataframe with two column
References
Kolde, R., Laur, S., Adler, P., & Vilo, J. (2012). Robust rank aggregation for gene list integration and meta-analysis. Bioinformatics, 28(4), 573-580.