Added complex manipulation ops and unit tests for added ops

[zaccharieramzi]

This PR adds:

• complex manipulation ops (going from and to complex-valued tensors)
• tests for complex manipulation ops
• tests for signal ops (fft3d and ifft3d)

The tests are passing in the current state, but I am going to suggest in a comment underneath some modifications to the current code.

[zaccharieramzi]

My suggestions:

• refactor the code for the fourier operators, with a base class sharing all the commonalities
• clarify the shape transposing in the docs

[EiffL]

Looks great, I've suggested a few modifications, let me know what you think and I can implement them if you agree :-)

[EiffL]

Ok I've made the executive decision of adding the code for the transpose inside the base class, the logic is this particular order of transpose is connected to the rest of the fft algorithm, so kind of awkward to have it alone in a different class

[EiffL]

LGTM, for some reasons the tests are not running automatically on github, I'm gonna try to ru them on my machine and after that it's all good

[EiffL]

ok, all good. Thanks @zaccharieramzi !