Reuse description of the NPV formula and Drogon model

[slawekszklarz]

Could be prettier to write a single model description and single objective function description and then include them in each tutorial case

[tup1985]

We do have the model_description page for that.
And I agree to break down the NPV formula into a separate page and include it, unless we will have different flavors of NPVs for different cases.