Switch to pytest for unit tests

[sfinkens]

Switch to pytest for unit tests, which fixes #87. Remove old end-to-end tests which require reference input/output data and were never run due to the incomplete test discovery.

[codecov[bot]]

Codecov Report

Merging #98 (2a641b9) into main (48a6567) will increase coverage by 2.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   67.06%   69.10%   +2.03%     
==========================================
  Files          34       34              
  Lines        2924     2887      -37     
==========================================
+ Hits         1961     1995      +34     
+ Misses        963      892      -71     

Impacted Files	Coverage Δ
pygac/tests/test_klm.py	93.84% <100.00%> (+93.84%)	:arrow_up:
pygac/tests/__init__.py	44.44% <0.00%> (-55.56%)	:arrow_down:
pygac/tests/test_init.py	58.33% <0.00%> (-33.34%)	:arrow_down:
pygac/tests/test_angles.py	78.26% <0.00%> (-17.40%)	:arrow_down:
pygac/tests/test_tsm.py	86.11% <0.00%> (-11.12%)	:arrow_down:
pygac/tests/test_calibrate_klm.py	90.00% <0.00%> (-8.00%)	:arrow_down:
pygac/tests/test_calibrate_pod.py	90.00% <0.00%> (-8.00%)	:arrow_down:
pygac/tests/test_slerp.py	92.06% <0.00%> (-6.35%)	:arrow_down:
pygac/tests/test_calibration_coefficients.py	91.04% <0.00%> (-5.98%)	:arrow_down:
pygac/tests/test_utils.py	91.17% <0.00%> (-5.89%)	:arrow_down:
... and 6 more

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