Introduced dedicated spel package and extracted value objects to encapsulate and express information about the node transformation in a more semantical way.
Moved a lot of the logic contained in the SpelExpressionTransformer into the value objects for cohesiveness and testability. Updated Sonargraph architecture model to reflect the new packages we've introduced.
Original pull request: #81.
Moved MongoDataIntegrityViolationException into core package to break up package cycle. Updated SOnargraph architecture description to capture issues more closely.
Updated Sonargraph architecture description to allow a dependency form the configuration layer to the GridFS layer. Dependency was introduced by c5a99b5b5e.
Updated Sonargraph architecture description file to include GridFS subsystem. Moved QueryMapper into convert subsystem. Polished MongoRepository interface. Let SimpleMongoRepository implement MongoRepository.
