biojson / jalview interoperability tests and documentation

The Jalview / biojson format specification requires documentation about what is preserved and what is lost when exporting from Jalview to json and back again. This could be included in Jalview as an additional page in the built-in/online help, or as a developer document in the docs directory. It is particularly important to note what is currently not preserved when exporting to biojson.

Tests should also be tagged which verify the preservation of currently supported jalview attributes when exported to json and imported again.