test suite for XBT functionality

Part of making this research reproducible is making sure the code runs and continues to run. This means running on multiple platforms and checking that continues to run as platforms and libraries evolve. A suite of tests of various sorts which can easily be run to check "code health" will be important before publishing the code and data.

tasks include
* use pytest for basic unit tests
* run tests using github actions

Start by reviewing the repository's existing notebooks and determine how pytest can cover the XBT functionality. Then define tests that can be run across platforms and add a GitHub Actions workflow; done means the suite runs successfully and provides an ongoing code-health check.

Written by the indexing model from the issue text.