Replace virtual with override wherever possible; eliminate needless virtual...

... for functions in final classes.

override is like const -- it's not necessary, but it helps the compiler to
catch mistakes.

There may be some overriding functions not explicitly declared virtual and I did
not identify such cases, in which I might also add override.

src/Resample.h

@@ -65,7 +65,7 @@ class Resample final
     @param outBufferLen How big outBuffer is.
     @return Number of output samples created by this call
    */
-   virtual int Process(double   factor,
+   int Process(double   factor,
                         float  *inBuffer,
                         int     inBufferLen,
                         bool    lastFlag,