* changes
* added actual tests for fit method
* precision
* syntax
* sampling not working
* sampling seems to work
* sampling not working again
* sampling works, tests to be removed
* replaced data with rmse
* replace pystan with cmdstanpy
* cleanup
* cleanup
* test for newton
* added support for multiple backends
* minor fixes
* fixed comment
* added support for --test-slow flag
* fixed import
* reverted style change
* specify backend based on env variable
* fixes
* PR fixes