Add test for ratio of increase in delay to stddev

Require that the ratio of the increase in delay from the minimum one in
the stats data register to the standard deviation of the offsets in the
register is less than maxdelaydevratio or the difference between
measured offset and predicted offset is larger than the increase in
delay. In the allowed delay increase is included also skew and maximum
clock frequency error.

maxdelaydevratio is 10.0 by default.

sources.h

@@ -145,6 +145,10 @@ extern double SRC_PredictOffset(SRC_Instance inst, struct timeval *when);
    currently held in the register */
 extern double SRC_MinRoundTripDelay(SRC_Instance inst);
 
+/* This routine determines if a new sample is good enough that it should be
+   accumulated */
+extern int SRC_IsGoodSample(SRC_Instance inst, double offset, double delay, double max_delay_dev_ratio, double clock_error, struct timeval *when);
+
 extern void SRC_DumpSources(void);
 
 extern void SRC_ReloadSources(void);