Group doctests into chunks

[alaindanet]

As a general trend within doctests in the package now, I think we could now transform all doctest lines with no expected output into chunks. Here for instance, instead of:

```jldoctest
julia> foodweb = FoodWeb([0 0; 1 0]);

julia> params = ModelParameters(foodweb);

julia> B0 = [0.5, 0.5];

julia> sol = simulate(params, B0);

julia> species_richness(sol; last = 10)
2.0

Prefer

```jldoctest
julia> foodweb = FoodWeb([0 0; 1 0]);
       params = ModelParameters(foodweb);
       B0 = [0.5, 0.5];
       sol = simulate(params, B0);
       species_richness(sol; last = 10)
2.0

I need to admit that this is my bad for not having figured we could do this within jldoctest chunks from the beginning. Now this associated poor style has become pervasive throughout the project :'( So, yeah, don't worry too much about tracking them down, we might do a whole pass over the docs to fix this everywhere later on ;)

Originally posted by @iago-lito in #87 (comment)

[iago-lito]

Addressed as part of #140.