MISC: Better Readme file

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: Remove DftRaw tests

Now that the Dft is on par with the (way faster) KissFFT wrapper, it is
no longer useful to test it and will be deprecated.
The class itself is still compilated in order to track changes in case
of common interface changes.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>