Utility to import/export FFTW plans

[pierrepaleo]

This PR adds a small utility for easily importing/exporting FFTW plans, called wisdom.

The wisdom data structure is saved in a numpy npz file, along with some "metadata" (eg. machine name). The rationale of this metadata is to avoid using plans created on different platforms - this can hamper performance.

The unit test is probably too simple. In theory, we should check that instantiating fftw.FFTW object is faster after importing wisdom (for the corresponding configuration) - but that can be tricky. Suggestions welcome :-)

Changelog: Add export_wisdom() and import_wisdom() in silx.math.fft.fftw