Assert tests

[jcapriot]

Changes check_derivative function to use assertions instead of returning True or False if the test passed or failed.
Will now raise an AssertionError if the tests fails, or return True if the test passed.

Also adds an assert_expected_order test function that doesn't use the OrderTest class. This is simpler to use with pytests's parameterized tests.

[codecov]

Codecov Report

Merging #310 (c3c2a54) into main (d5a27bf) will increase coverage by 0.02%.
The diff coverage is 81.53%.

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
+ Coverage   85.22%   85.24%   +0.02%     
==========================================
  Files          38       38              
  Lines       11284    11301      +17     
==========================================
+ Hits         9617     9634      +17     
  Misses       1667     1667              

Impacted Files	Coverage Δ
discretize/tests.py	80.45% <81.53%> (+1.33%)	⬆️

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