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.