[1d3d] Copy privars only once and use the correct numeric epsilon for the extended source stencil

Merged Timo Koch requested to merge bugfix/use-correct-epsilon-for-extended-source-stencil into master
  • Copying privars only once for efficiency.
  • Using the correct epsilon fixes a bug (bad Jacobian approximation) for zero initial data and/or when the epsilon is given in the input file

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