Make subfunctions for trophic structure analysis

[alaindanet]

          I would include my functions in this file, but probably not in this function, IMO it would be too much to put into one functions and I like when functions stay specific. 

The best for me would be to create a new separated function (e.g. trophic_class).
Also I was wondering if we could split even more these functions, I mean have a separated max_trophic_level(), mean_trophic_level(), etc. We could keep this function that aggregates several metrics, but I think we also wants to expose specific functions that compute each of these metrics for the user who would like to compute only one metric, no?

Originally posted by @ismael-lajaaiti in #87 (comment)