Implement second derivative for Hermite splines

[faymanns]

In the original version, the CubicHermite and the ExponentialHermite basis functions where implemented as not twice differentiable, but they are in fact twice differentiable. They are just not C2. Since some of the non-Hermite basis functions are also not C2 but we have implemented the second derivative, we should also do this for the Hermite splines.

Even though the second derivatives are not continuous, having them will be useful to compute curvature and bending energy.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.48%. Comparing base (9583f0b) to head (edd5d10).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
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- Coverage   98.49%   98.48%   -0.01%     
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  Files           3        3              
  Lines         530      528       -2     
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- Hits          522      520       -2     
  Misses          8        8              

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