addGeneSetMetadata.RdThis 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, ...)
| x | a   | 
    
|---|---|
| meta | a   | 
    
| ... | not used yet  | 
    
the updated GeneSetDb object x.
TODO: this should be a setReplaceMethod, Issue #13 https://github.com/lianos/multiGSEA/issues/13