Fix bug in Matern12 kernel

[thomaspinder]

This PR resolves #118 by fixing the first-order Matérn kernel

Pull request type

Please check the type of change your PR introduces:

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

What is the current behavior?

Currently an incorrect 0.5 constant in the kernel's __call__ function.

Issue Number: #118

What is the new behavior?

Constant has now been removed to be consistent with the literature.

Other information

[codecov]

Codecov Report

Merging #119 (5e20e1a) into master (babf010) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #119   +/-   ##
=======================================
  Coverage   98.95%   98.95%           
=======================================
  Files          13       13           
  Lines         961      961           
=======================================
  Hits          951      951           
  Misses         10       10           

Flag	Coverage Δ
unittests	98.95% <100.00%> (ø)

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Impacted Files	Coverage Δ
gpjax/kernels.py	98.68% <100.00%> (ø)

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