network models can be disconnected

[gabrielgellner]

This is a known issue for these kinds of models, should our code return such a model? Or should it rerun until a fully connected model is given?

Also it would be worth to do some tests on how the number of connected models scales with S and C and model type.

[gabrielgellner]

I have a check now based on looking at the DiGraph from LightGraphs to check for connectance in the niche_network. Need to think about doing this for the other models that would require it. Also add some tests.