Add derivative example

[yasahi-hpc]

Resolves #49

[rb214678]

When comparing the derivative of a function using FFT-based methods and the one computed with the analytical solution, in the Python script you use np.testing.assert_allclose with a tolerance. In the C++ code you do it "by hand" with some loop. Do you need these checks to be identical ? If yes it is the case ?

[yasahi-hpc]

Thank you for the review.

When comparing the derivative of a function using FFT-based methods and the one computed with the analytical solution, in the Python script you use np.testing.assert_allclose with a tolerance. In the C++ code you do it "by hand" with some loop. Do you need these checks to be identical ? If yes it is the case ?

They are not exactly the same.
I am currently considering to introduce an additional package for testing like numpy.
Once I introduce it, I can use that for the sanity check in this example.
However, it is out of the scope of this PR, so I keep it as is.