Move params to the first slot of each function, class, etc.

[daniel-dodd]

This PR moves the params argument to the first slot of each function, to match the convention of Haiku and Flax.

Also, cleans up some of the tests, add typing, for readability and adds testing of abstract classes. Some test files e.g. 'test_gps.py` need cleaning up, in future.

• Bugfix
• Feature
• [ X] Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

[codecov]

Codecov Report

Merging #139 (afa13cf) into v0.5_update (9ee096a) will increase coverage by 0.00%.
The diff coverage is 98.14%.

@@             Coverage Diff              @@
##           v0.5_update     #139   +/-   ##
============================================
  Coverage        98.32%   98.32%           
============================================
  Files               15       15           
  Lines             1372     1377    +5     
============================================
+ Hits              1349     1354    +5     
  Misses              23       23           

Flag	Coverage Δ
unittests	98.32% <98.14%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gpjax/kernels.py	95.08% <84.61%> (ø)
gpjax/gps.py	100.00% <100.00%> (ø)
gpjax/likelihoods.py	100.00% <100.00%> (ø)
gpjax/mean_functions.py	100.00% <100.00%> (ø)
gpjax/variational_families.py	100.00% <100.00%> (ø)
gpjax/variational_inference.py	97.77% <100.00%> (+0.02%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more