Change to use matplotlib's lim for zooming

closes #118 (closed)

When zooming before, downsample need to be recalculated and replot. With the reducing of data points, downsample isn't necessary anymore. SOHView now can use lim for zooming.

• Remove set_lim in PlottingWidget's subclasses.
• Recalculate total points for plot_time_dots, plot_multi_color_dots and plot_up_down_dots. (They were set when recreating axis before)
• Remove the condition to handle 'ax' or 'ax_wf' are already in c_data in SOHWidget and WaveformWidget. This case only takes place when a channel is replotted.

sohstationviewer/view/plotting/plotting_widget/plotting.py

@@ -181,6 +181,8 @@ class Plotting:
             sample_no_pos=[0.25, 0.75],
             chan_db_info=chan_db_info, linked_ax=linked_ax)
 
+        # x_bottom, x_top are the times of data points to be displayed at
+        # bottom or top of the plot
         ax.x_bottom = np.array(points_list[0])
         ax.x_top = np.array(points_list[1])