This function adds/updates columns entries in the geneSets(gdb) table.
If there already are defined meta values for the columns of meta in x,
these will be updated with the values in meta.
addGeneSetMetadata(x, meta, ...)the updated GeneSetDb object x.
TODO: should this be a setReplaceMethod, Issue #13 (?) https://github.com/lianos/multiGSEA/issues/13