Fepois: Add support for `predict` with `newdata` argument

## Context

Currently, the `newdata` argument is disabled for `Fepois.predict`. Unfortunately, I do not recall why... 😅 

## To do

- Enable the `newdata` argument for `Fepois.predict(). 
- Clean up ValueError when trying to run Fepois.predict() with `newdata` in `tests/test_errors.py`.
- Add unit tests to `tests/test_vs_r::test_single_fit`.