Check the published implementation of the C3 C4 competition model

[davidorme]

I think there have been some changes in the C3C4Competition between first implementation and publication to reduce the complexity of the tree cover setup. Look at aligning this implementation to the published method.

Although, in practise C3C4Competition is more of a workflow than a coherent Class. Possibly consider exploding into separate functions/classes that can be swapped rather than a monolithic method that is a hostage to fortune.