Incorporate JaxLinOp with GPJax

[daniel-dodd]

This PR incorporates JaxLinOp with GPJax.

(Still need to update documentation).

[codecov-commenter]

Codecov Report

Merging #154 (b41f7e4) into master (bf10dfd) will decrease coverage by 0.79%.
The diff coverage is 89.72%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
- Coverage   98.32%   97.53%   -0.80%     
==========================================
  Files          15       15              
  Lines        1377     1339      -38     
==========================================
- Hits         1354     1306      -48     
- Misses         23       33      +10     

Flag	Coverage Δ
unittests	97.53% <89.72%> (-0.80%)	⬇️

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

Impacted Files	Coverage Δ
gpjax/kernels.py	93.22% <57.14%> (-1.86%)	⬇️
gpjax/gaussian_distribution.py	84.70% <84.70%> (ø)
gpjax/gps.py	100.00% <100.00%> (ø)
gpjax/likelihoods.py	100.00% <100.00%> (ø)
gpjax/variational_families.py	100.00% <100.00%> (ø)
gpjax/variational_inference.py	97.80% <100.00%> (+0.02%)	⬆️

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[thomaspinder]

Looks good to me. A few minor comments and then we can merge.