expose an API, not just the app

An app can not load a Julia package. At least not conveniently. Suppose someone wanted to measure the error for `logit` from [LogExpFunctions.jl](https://github.com/JuliaStats/LogExpFunctions.jl), currently they would need to redefine `logit` when calling the app on the command line.

Thus it would be preferable to expose an actual API.

Start by reviewing the command-line app entry point and how it currently accepts a function. Use the LogExpFunctions.jl `logit` example to define the API boundary and confirm that a Julia package can call the error measurement without redefining the function. Done means the functionality is exposed as a usable API in addition to the app.

Vom Indexierungsmodell aus dem Issue-Text verfasst.