Improved constructors for all `AbstractMicrobe` subtypes and `ABM` to minimize verbosity and boilerplate code.

[mastrof]

Major

• [x] #76
• [ ] Smarter external constructors for microbe and motility types

  Minor

• [x] #32
• [x] #14
• [x] #66

[mastrof]

On top of allowing a few extra keywords (mostly to tap into all the Agents.jl functionalities), it is probably convenient to start introducing functions that act in-place on model, after it is defined.

In particular, model.properties can be modified in-place to introduce extra properties. The update_model! function could become a model property itself; then for example calling a function like encounters!(model; kwargs...) could add all the required properties, and also modify update_model! to automatically include the evaluation of encounters.