Remove unneeded header inclusion.
Add additional `delta` method to array iterators.
Remove complicated implementation for differential functions.
Simplify the gradient implementation.
Simplify the Laplacian implementation.
Simplify the implementation of the Hessian computation.
Change the signatures of convenience functions.
Add unit tests for the `Differential` submodule.
Update `CMakeLists.txt`.
Merge pull request #129 from DO-CV/maint-test-differential-submodule

[MAINT] Add unit tests for the `Differential` submodule.