Tidy up DataSources.groovy

This doesn't resolve #67 as such - more of a tidy up.

I did try a few things:
- run up 6-nec-mel, connect the AATAMS app to a local DB.  Stopped DB, get errors from web-app, start DB - all good.

So in general, I think connection problems are being handled.

Probably @julian1's idea of cleaning up idle connections on the DB will have more impact on resolving this issue than anything else.