Prevent crash when array is not a numpy array.

@Catriix pointed out that the np.where command only works with
numpy arrays so converting the RR sample pos into an np array.

hrv.py

@@ -116,6 +116,7 @@ def SDANN(self, rr_samples, average_period=5.0, normalise=False):
 
         average_period_samples = int(self.fs*average_period*60)
         average_rr_intervals = []
+        rr_samples = np.array(rr_samples)
 
         sections = int((np.max(rr_samples)/average_period_samples)+0.5)