Prevent f64 default

[thomaspinder]

Make float32 the default type, not f64.

Pull request type

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

What is the current behavior?

Currently we enforce float 64. However, despite giving more stable matrix inversions, this limits reproducibility.

Issue Number: #128

What is the new behavior?

Default to float32 and set float64 in notebooks.

Other information

[codecov]

Codecov Report

Merging #129 (fd2f974) into v0.5_update (8b02848) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           v0.5_update     #129      +/-   ##
===============================================
- Coverage        98.27%   98.27%   -0.01%     
===============================================
  Files               15       15              
  Lines             1333     1331       -2     
===============================================
- Hits              1310     1308       -2     
  Misses              23       23              

Flag	Coverage Δ
unittests	98.27% <ø> (-0.01%)	⬇️

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

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