Add Base.:+ and Base.:* for kernels

[JoshuaLampert]

Add convenience functions to add and multiply kernels. Wrap them automatically as SumKernel and ProductKernel. The same for TransformationKernel and Base.:∘ is more involved since we would need to infer the input dimension of the transformation because this is the new dimension Dim of the kernel (the output dimension of the transformation is the input dimension of the base kernel and thus known).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.71%. Comparing base (a72dbdf) to head (3893b62).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #45   +/-   ##
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  Coverage   98.71%   98.71%           
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  Files          32       32           
  Lines         933      935    +2     
=======================================
+ Hits          921      923    +2     
  Misses         12       12           

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