0.6.1

Maintenance release with some minor breaking changes.

• [core] Fix spatial derivatives when using mode="bspline" or mode="gaussian" (#135).
• [core] Rename SpatialDerivativeKeys all() and unmixed() argument ndim to spatial_dims (#132).
• [data] Fix exception when passing a FlowFields batch tensor to F.grid_sample() (#134).
• [losses] Add spacing option to flow field loss modules (#136).
• [spatial] Reverse stride order when subsampling DenseVectorFieldTransform (#133).