ENSCORESW-941: only store an entry in the mapping_set table if there are two databases to compare
ENSCORESW-941: only store an entry in the mapping_set table if there are two databases to compare

mjg17/ensembl,muffato/ensembl,danstaines/ensembl,at7/ensembl,muffato/ensembl,at7/ensembl,mjg17/ensembl,muffato/ensembl,Ensembl/ensembl,muffato/ensembl,at7/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,Ensembl/ensembl,at7/ensembl,danstaines/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,Ensembl/ensembl