Support `sampling` kwarg for `distance_transform_edt` (take pixel/voxel sizes into account)

[grlee77]

Please review/merge #406 before this one as those changes are also present here.

This PR adds support for anisotropic pixel/voxel shape to distance_transform_edt via the sampling kwarg. The changes involve just making variants of the kernels involving distance computations that do floating-point instead of integer-valued computations so that scaling by the sample spacings can be taken into account.