Fix C4189 and C4100 Warnings

C4189 is 'Local variable initialised but not used'
C4100 is 'Unreferenced parameter'

Fixed some other warnings, e.g. about empty if, too.

src/FFT.cpp

@@ -585,6 +585,8 @@ void DerivativeOfWindowFunc(int whichFunction, size_t NumSamples, bool extraSamp
       // There are deltas at the ends
       const double multiplier = 2 * M_PI / NumSamples;
       static const double coeff0 = 0.54, coeff1 = -0.46 * multiplier;
+      // TODO This code should be more explicit about the precision it intends.
+      // For now we get C4305 warnings, truncation from 'const double' to 'float' 
       in[0] *= coeff0;
       if (!extraSample)
          --NumSamples;