New library for math...

... note the swap of target_link_libraries lines in src/CMakeLists.txt,
needed to build at least on macOS, becuase FFT.h must be looked up first in
lib-math, not in lib-src/twolame

Also making a dependency cycle of SampleFormat and Dither!  But we will tolerate
that within one small library.

libraries/lib-math/InterpolateAudio.h

@@ -0,0 +1,41 @@
+/**********************************************************************
+
+  Audacity: A Digital Audio Editor
+
+  InterpolateAudio.h
+
+  Dominic Mazzoni
+
+*******************************************************************//*!
+
+\file Matrix.h
+\brief General routine to interpolate (or even extrapolate small amounts)
+ audio when a few of the samples are bad.  Works great for a few
+ dozen bad samples, but not so well with hundreds.  Uses the
+ least-squares autoregression (LSAR) algorithm, as described in:
+
+ Simon Godsill, Peter Rayner, and Olivier Cappe.  Digital Audio Restoration.
+ Berlin: Springer, 1998.
+
+ This is the same work used by Gnome Wave Cleaner (GWC), however this
+ implementation is original.
+
+*//*******************************************************************/
+
+#ifndef __AUDACITY_INTERPOLATE_AUDIO__
+#define __AUDACITY_INTERPOLATE_AUDIO__
+
+#include <cstddef>
+
+// See top of file for a description of the algorithm.  Interpolates
+// the samples from buffer[firstBad] through buffer[firstBad+numBad-1],
+// ignoring whatever value was there previously, and replacing them with
+// values determined from the surrounding audio.  Works best if the bad
+// samples are in the middle, with several times as much data on either
+// side (6x the number of bad samples on either side is great).  However,
+// it will work with less data, and with the bad samples on one end or
+// the other.
+void MATH_API InterpolateAudio(float *buffer, size_t len,
+                                       size_t firstBad, size_t numBad);
+
+#endif // __AUDACITY_INTERPOLATE_AUDIO__