Use numpy testing framework and relative differences

[rgieseke]

Also adds a test for y in reference data and fixes a bug with y-values in pr.

[rgieseke]

All tests have passed locally as well.

platform linux -- Python 3.11.10, pytest-8.3.4, pluggy-1.5.0
configfile: pyproject.toml
collected 42 items                                                                                                                                                                                                                           

tests/test_detrend.py .........                                                                                                                                                                                                        [ 21%]
tests/test_preprocessing.py ..                                                                                                                                                                                                         [ 26%]
tests/test_vendored_ssa.py ..............................                                                                                                                                                                              [ 97%]
tests/test_version.py .                                                                                                                                                                                                                [100%]

=== 42 passed