Implement the Earth4All model

[natema]

In order to include in our list of models of those developed by the Club of Rome, we only miss World1 and Earth4All (E4A).
This issue is about adding the E4A model.

[natema]

Branch e4a contains some WIP towards providing the implementation.

[natema]

The porting is making progress but we're being slowed down by #133 ☹️

[natema]

We should be able to merge in dev a first version of e4a soon.

[natema]

The implementation of Earth4All has been successfully completed and is now available at in a separate repo.
We will discuss whether we want to organize the models as external repos like Mimi.jl. The latter had been already discussed in the past, and that option looks more meaningful to me and @piluc now.
Until then, keeping this issue open can be misleading, so I'll close it and we'll reopen it if we decide to stick with past decisions.