Refactor high level constructor interface

[radka-j]

What is the current workflow and can this be improved?

Eventually we want to do inference over the a/b parameters, number of planktons etc. At the moment, this is only possible by creating a new constructor class. What we really want is to create the BGC abstract type with the appropriate emergent parameters and methods (with/without default values) and then create a function that returns a concrete instance given the high level, allometric parameters (including N and diameters).