Change way to access monitoring info in dl1 file

The former approach np.array(f.root[].cols.charge_mean) results now  in an attempt to allocate a huge array (instead of just 2x2x1855), and hence a crash. 
No idea when or why the change of behaviour happened - could it be because of some change in pytables?

lstchain/calib/camera/pixel_threshold_estimation.py

@@ -24,10 +24,10 @@ def get_bias_and_std(dl1_file):
     """
     with tables.open_file(dl1_file) as f:
         ped = f.root[dl1_params_tel_mon_ped_key]
-        ped_charge_mean = np.array(ped.cols.charge_mean)
-        ped_charge_std = np.array(ped.cols.charge_std)
+        ped_charge_mean = ped.col('charge_mean')
+        ped_charge_std = ped.col('charge_std')
         calib = f.root[dl1_params_tel_mon_cal_key]
-        dc_to_pe = np.array(calib.cols.dc_to_pe[ORIGINAL_CALIBRATION_ID])
+        dc_to_pe = calib.col('dc_to_pe')[ORIGINAL_CALIBRATION_ID]
         ped_charge_mean_pe = ped_charge_mean * dc_to_pe
         ped_charge_std_pe = ped_charge_std * dc_to_pe