Document in more detail what the scenario interpolation is actually doing.

[aaccensi]

Context

Right now we have documentation in the API section about the scenario interpolation only focusing pretty much on how to use it.

Current shortcoming

There is no documentation explaining what the "interpolation" process actually entails.

Proposed enhancement

We should explain also interpolation from a conceptual point of view, what happens to the inputs, to the curves, shortcomings or advantages of our method...

Success criteria

• Add the conceptual explanation
• Test that everything still works