Give grouped data from functions

[parzhitsky]

Currently, there is a way of grouping currency data by certain criteria (country name, currency code, etc.). The way it is done (predefine an environment variable and invoke the postinstall script) is at least a little bit unusual, and at most, it has some minor problems (like recalculating the grouped data at each npm i or npm ci).

To make the functionality more idiomatic, it might make sense to define a couple of utility functions that would return the grouped data as a return value.

[parzhitsky]

It might make sense to define a couple of utility functions

There are several considerations:

• No, it wouldn't make sense, the current approach is sufficient.
• Yes, the functions would produce grouped data as a side-effect (i.e., we are still building a data-grouped-by-*.json file, but there is a new trigger for that now - a correspondent function call). The file has to then be imported/read/parsed manually.
• Yes, the functions would produce data as a return value, possibly also producing the file as a side-effect.
• Yes, the functions would be pure; they would take the data as an argument and return the grouped data exclusively through RAM, not through a file.
• Yes, there should be one function that would take the criterion(-a) as an argument.