Small explanation on the feedback fixed delay
Not working yet (except if both delays are equal) and not enough tests.
Used the variable universal line implementation as reference. Although it may be more complex for the processor, it is simpler to code, as there are only 2 loops instead of a magnitude more if using less memory.

The tests still need to be updated.
Tests for no mixing channels OK
Merge branch 'feature/StereoUniversalFixedDelayLine' into develop