updates select samples to avoid pulse not recorded due to Rpi lag

ohmpi/hardware_system.py

@@ -296,9 +296,9 @@ class OhmPiHardware:
         x = []
         for pulse in range(int(max(self.readings[:, 1]))):
             v = np.where((self.readings[:, 1] == pulse))[0]
-
-            t_start_pulse = min(self.readings[v, 0])
-            x.append(np.where((self.readings[:, 0] >= t_start_pulse + delay) & (self.readings[:, 2] != 0) & (
+            if len(v) > 0:  # to avoid pulse not recorded due to Raspberry Pi lag...
+                t_start_pulse = min(self.readings[v, 0])
+                x.append(np.where((self.readings[:, 0] >= t_start_pulse + delay) & (self.readings[:, 2] != 0) & (
                         self.readings[:, 1] == pulse))[0])
         x = np.concatenate(np.array(x, dtype='object'))
         return x