Extend runs test

Double the number of samples that are used in the runs test. E.g. with 64
samples in regression the runs test will be tried over the 64 samples and
up to 64 previous samples. The minimum number of samples in now 4.

This improves the response with low-mid jitters by about 50%.

regress.h

@@ -66,6 +66,10 @@ extern double RGR_GetTCoef(int dof);
 
 extern double RGR_GetChi2Coef(int dof);
 
+/* Maximum ratio of number of points used for runs test to number of regression
+   points */
+#define REGRESS_RUNS_RATIO 2
+
 /* Return a status indicating whether there were enough points to
    carry out the regression */
 
@@ -77,6 +81,9 @@ RGR_FindBestRegression
                                    less reliable) */
  
  int n,                         /* number of data points */
+ int m,                         /* number of extra samples in x and y arrays
+                                   (negative index) which can be used to
+                                   extend runs test */
 
  /* And now the results */