Remove allocations in the spline_interpolation function

[andrewwinters5000]

This modifies the four variants of the spline_interpolation function to use static arrays and avoid allocations. Once merged and released this will address the failing allocation test in TrixiShallowWater PR #102.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.53%. Comparing base (9205512) to head (1df5c64).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
+ Coverage   96.48%   96.53%   +0.05%     
==========================================
  Files           9        9              
  Lines         483      491       +8     
==========================================
+ Hits          466      474       +8     
  Misses         17       17              

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