Adds constant strategy

ohmpi/hardware_system.py

@@ -436,6 +436,7 @@ class OhmPiHardware:
             diff_vab = np.inf
             if strategy == 'constant':
                 tx_volt = vab_max*.9
+                strategy = 'vmax'
             if strategy == 'vmax':
                 while (k < n_steps) and (diff_vab > diff_vab_lim) and (vab_list[k] < vab_max):
                     vabs = []