add automated tests for main methods, add domain dataset(s)

[PetrKorab]

Add automated tests covering the main Metasyn's methods, in line with JOSS guidelines. Also, please add tests for at least one domain dataset where you show the benefits of the programme. Make the data available to the reviewers, if possible. Thanks!

[qubixes]

@PetrKorab Thanks for your comments. Could you elaborate in more detail which automated tests you are missing? We currently have >90% coverage and they are tested automatically with GitHub actions. Which main methods do you find untested?

[PetrKorab]

@qubixes Please add here a link to a folder with automated tests (JOSS submission) showing Metasyn's functionality on at least one other domain dataset (except the Titanic one). Thanks.

[qubixes]

The link to our test folder is https://github.com/sodascience/metasyn/tree/develop/tests for our development branch. I have added a medical dataset in #330, would that suffice as another domain dataset to you?