Added some doc about possible solutions for AATAMS' security architecture.
tiff -> png to make github happy.
New option: geotools data source.
Update security_in_db.md

Added some more pros and cons to "AATAMS GeoTools Data Source" section.
Added new architecture option.

It became apparent that none of the others previously suggested would be feasible.
Merge pull request #107 from aodn/security_in_db

Security in db