Restructure tests

This merge request restructures the test folder to match the structure of the source code package. I did not change any of the tests, because line by line code changes would be hard to make out in the diff.

setup.cfg

@@ -10,7 +10,7 @@ addopts =
     --ignore test/core/example_test.py
     --ignore test/gw/sample_from_the_prior_test.py
     --ignore test/gw/plot_test.py
-    --ignore test/core/sampler_test.py
+    --ignore test/core/sampler/sampler_run_test.py
 
 [metadata]
 license_file = LICENSE.md