MessageType extendability

• Refactoring to change message type specific code from hard-code into Spring configuration to make it possible to add new message types in extending projects. The fi.fmi.avi.model.AviationCodeListUser.MessageType enum is replaced by fi.fmi.avi.model.MessageType class. New message types can now be created in runtime code using MessageType constructor. In practice this means that '==' comparisons with MessageType no longer work reliably, equals should be used instead. The message type also can no longer be used as a casing condition in switch statements.
• GTSExchangeFileInfo now produces timestamps with second precision by default.