Examples 3D+time

[cyrilmory]

This PR moves the code generating the data for 3D+time tests and examples into a helper function, which can be reused.
The changes are currently applied only to rtkfourdroostertest.cxx, but can be propagated to the other 3D+time tests.
The PR also adds the 4D conjugate gradient C++ example, using the helper function to generate the data.

If it is fine for you, I will add the other examples as additional commits on this same PR.

[SimonRit]

Thanks. I'm no so fond of having source files out of the example folder, I prefer to have them self contained. I would prefer to have one example to generate the data, a copy of which could be provided on girder, and then other examples to use the generated data. Would that be doable?

[cyrilmory]

Yes, that's doable. And would you keep the helper function for the tests, or get rid of it entirely ?

[SimonRit]

Yes, that's doable. And would you keep the helper function for the tests, or get rid of it entirely ?

You can keep it for the tests if it's useful, we've done it with other codes.

[cyrilmory]

Is is better now ? If so, I'll proceed with the other 3D+time examples

[SimonRit]

I have rebased and then made several modifications to this PR:
https://github.com/RTKConsortium/RTK/compare/bb76a114c163591771c2300ef18b38554795bb10..9163aab7a33048b22e97914cff522f7a6d3580b5

@axel-grc Can you please take over and

• Check that the test run
• Add images in the documentation
• Translate the examples to Python
  ? Thanks!