Interpolator for gradient calculation is linear, i.e. not using the interpolator set for the cost function

Trying to solve InsightSoftwareConsortium/ITK#6352 I found that the interpolator that is used to calculate the gradient is always linear
https://github.com/InsightSoftwareConsortium/ITKMinimalPathExtraction/blob/2ff1ab58f30842ac57d76c8bc9462fcbc7523e67/include/itkPhysicalCentralDifferenceImageFunction.h#L86
and not using the interpolator that is (possibly) set for the cost function
https://github.com/InsightSoftwareConsortium/ITKMinimalPathExtraction/blob/2ff1ab58f30842ac57d76c8bc9462fcbc7523e67/include/itkSingleImageCostFunction.h#L105-L107
It seems the cost function interpolator is used in case iterateNeighborhoodOptimizer (which uses [`GetValue`](https://github.com/InsightSoftwareConsortium/ITKMinimalPathExtraction/blob/2ff1ab58f30842ac57d76c8bc9462fcbc7523e67/include/itkSingleImageCostFunction.hxx#L86-L89)) is employed
https://github.com/InsightSoftwareConsortium/ITKMinimalPathExtraction/blob/2ff1ab58f30842ac57d76c8bc9462fcbc7523e67/include/itkSingleImageCostFunction.hxx#L102
but not when the gradient is computed (which uses [`GetDerivative`](https://github.com/InsightSoftwareConsortium/ITKMinimalPathExtraction/blob/2ff1ab58f30842ac57d76c8bc9462fcbc7523e67/include/itkSingleImageCostFunction.hxx#L114-L118))
https://github.com/InsightSoftwareConsortium/ITKMinimalPathExtraction/blob/2ff1ab58f30842ac57d76c8bc9462fcbc7523e67/include/itkSingleImageCostFunction.hxx#L137

This however does not seem to be the issue of InsightSoftwareConsortium/ITK#6352 (where a linear interpolator was used anyway), so posting as an issue on its own.