Helpers functions for connector instantiation

[pkrog]

• Make a default biodb instance.
• Implement a newConn() helper function that uses this default instance for calling defaultInstance$getFactory()$createConn().
• Implement a getConn() helper function that uses this default instance for calling defaultInstance$getFactory()$getConn().
• Is it possible to define dynamically at loading time an helper function for each connector class (e.g.: keggGlycan()) along with its documentation ? If not, can we define variables attached to the biodb namespace: biodb::keggGlycan ?