Stop using Cython (pseudo_spect_cy.pyx)

Created originally on Bitbucket by paugier (Pierre Augier)

Was already merged in Bitbucket before import, marked as merged by the import user

It should fix this problem https://bitbucket.org/fluiddyn/fluidsim/issues/28/installing-fluidsim-in-miniconda3-on

A Cython extension was built but it wasn’t used any longer by default.

This PR makes fluidsim a pure Python code (with bits compiled by Transonic / Pythran).

It decreases the installation and maintenance issues.

It’s going to be easier to improve our time stepping code.

Without Pythran, it leads to a small performance reduction, but anyway, for performance, Fluidsim needs Pythran.