Trophic bugfixes

[mjsmith037]

Several bugs fixes in calculating trophic levels and omnivory

This pull request will hopefully address some inconsistencies and bugs in trophiclevels.jl and omnivory.jl.

Specifically:

• The functions for calculating trophic level and omnivory use an unconventional convention for trophic link direction. This means that in several places out-degree is used rather than in-degree, and in others there is inconsistent use of degree
• Related, in some places iteration is over predators rather than prey
• The calculation of shortest paths associated with trophic level calculation uses the maximal shortest path rather than the minimal as expected
• the calculation of omnivory uses shortest paths rather than true trophic level

Related issues

#202

Checklist

• The code is covered by unit tests
  • Additional cases or module in test/...
  • Relevant lines in test/runtests.jl
• The code is documented
  • Docstrings written for every function
  • If needed, additional lines in docs/src/...
• All contributors have added their name and affiliation to .zenodo.json

Pinging

Pinging @tpoisot

[tpoisot]

I have merged this into #206 - let's pick up from there!