Improve tests

[vnmabus]

There are a couple of ideas for improving the tests in this package.

For the tests, I think that the ideal thing would be to have each dataset as both binary and ASCII. Then it should be possible, maybe using a Pytest fixture, to pass the same tests for both versions, ensuring that all functionality work for both implementations.

Originally posted by @vnmabus in #34 (comment)

I think it could also be useful to include in the repo an R script that generates all those test data files.

Originally posted by @trossi in #34 (comment)