implement: add metadata to extracted data as a separate file

[eackermann]

We need several pieces of information, most critically the sampling rate of recordings / processed data, but also data types.

One option could be to generate and use info.json files, similar to those found in https://github.com/tridesclous/tridesclous_datasets

An example of an info.json file might look like this:

{
    "dtype": "float32",
    "sample_rate": 15000.0,
    "shape": [-1, 4]
}

[jchutrue]

Should include epochs as well

[jchutrue]

Well, epochs aren't really metadata, so never mind