Create the Application class

[RobinPicard]

Addresses issue #1346

The class will actually be called Application instead of Outline, but it will otherwise implement the interface suggested in the linked issue. We additionally remove the functions module.

[rlouf]

Could we add a with_backend method that allows a user to change how a model is going to be run? And lazily load the model and compile lazily so this does not happen when the Macro is instantiated, only when it is used?

The idea is to eventually have people only pass a string with the model name.

[rlouf]

Supersedes #1364

[rlouf]

Wondering if we should simply call it Application

[RobinPicard]

I prefer Application. My issue with Macro is that it sounds too much like it's creating a filled-out template that still needs to be run. Is there a risk we will want to use the word "Application" for something else in the future though?

[rlouf]

Let's use Application for now

[rlouf]

We had an offline discussion where we decided to pass the model as an argument when calling the application. This makes Application object more re-usable.