Fix naming and add gradients

[LudwigBoess]

No description provided.

[codecov]

Codecov Report

Merging #12 (75234b9) into master (3d91bd5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #12   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          383       383           
=========================================
  Hits           383       383           

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

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Impacted Files	Coverage Δ
src/wendland/C6.jl	100.00% <ø> (ø)
src/SPHKernels.jl	100.00% <100.00%> (ø)

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[LudwigBoess]

Closing this for now. Will revisit once #13 and #14 are resolved.