Nutrient intake

[evadelmas]

:sparkles: New feature: Nutrient intake model

This PR includes some changes in the code architecture:

• new producer_growth field in ModelParameters: this field contains the model for producer growth and the corresponding parameters. For now it can be one of either LogisticGrowth (with parameters K and α) or NutrientIntake (with all corresponding parameters)
• argument K (carrying capacity) moved from environment to producer_growth
• field producer_competition removed, α moved to producer_growth

[evadelmas]

To do:

• [x] Remove K from environment
• [x] Fix the extinction callback
• [x] Resolve conflicts
• [ ] Fix the existing tests
• [x] Add new tests
• [x] Document
• [ ] Use case
• [ ] House cleaning

[evadelmas]

I just realized that the extinction callback needs to be adjusted to avoid monitoring nutrients (added to the todo list above).

[iago-lito]

Wops, sorry @evadelmas I did not realize that you issued comments here before I finished my review. I'm addressing them rn :)

[iago-lito]

This feature continues in #124.

[iago-lito]

Closed by #124.