`produce_or_load` doesn't recognize change in the function producing the data.

[Datseris]

Given how produce_or_load works, because it deduces the name solely on the config argument, it doesn't care about the function f. This means, that if I produce some data, then change f, then the data will be loaded instead of re-produced.

This is not good, and we should probably change it. Question is, how? If the file is saved as .jld2, then we can add an internal field like _drwatson_f_hash that stores a "hash" of the function.

Now, how do you create a "hash" out of a function, in a way that makes sense?