Move example from gnc_numeric.h to a separate Doxygen file.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@13751 57a11ea4-9604-0410-9ed3-97b8803252fd

src/doc/gnc-numeric-example.txt

@@ -0,0 +1,48 @@
+/** \page gncnumericexample gnc_numeric Example
+
+\section example EXAMPLE
+
+The following program finds the best ::gnc_numeric approximation to
+the \a math.h constant \a M_PI given a maximum denominator. For
+large denominators, the ::gnc_numeric approximation is accurate to
+more decimal places than will generally be needed, but in some cases
+this may not be good enough. For example,
+
+@verbatim
+    M_PI                   = 3.14159265358979323846
+    245850922 / 78256779   = 3.14159265358979311599  (16 sig figs)
+    3126535 / 995207       = 3.14159265358865047446  (12 sig figs)
+    355 / 113              = 3.14159292035398252096  (7 sig figs)
+@endverbatim
+
+@verbatim
+#include <glib.h>
+#include <qof.h>
+#include <math.h>
+
+int
+main(int argc, char ** argv)
+{
+  gnc_numeric approx, best;
+  double err, best_err=1.0;
+  double m_pi = M_PI;
+  gint64 denom;
+  gint64 max;
+
+  sscanf(argv[1], "%Ld", &max);
+  
+  for (denom = 1; denom < max; denom++)
+  {
+    approx = double_to_gnc_numeric (m_pi, denom, GNC_RND_ROUND);
+    err    = m_pi - gnc_numeric_to_double (approx);
+    if (fabs (err) < fabs (best_err))
+    {
+      best = approx;
+      best_err = err;
+      printf ("%Ld / %Ld = %.30f\n", gnc_numeric_num (best),
+              gnc_numeric_denom (best), gnc_numeric_to_double (best));
+    }
+  }
+}
+@endverbatim
+*/