tps tab bug

• is_plotting_tps isn't reset to false which results in giving message "Already Working" and can't plot any more data set after the first one.
• Error when click stop while plotting tps

Traceback (most recent call last):
  File "/Users/ldam/Documents/GIT/sohview/sohstationviewer2/sohstationviewer/view/main_window.py", line 783, in stop
    tps_widget = self.tps_dlg.plotting_widget
AttributeError: 'TimePowerSquaredDialog' object has no attribute 'plotting_widget'

• Error when continue to plot tps again

Traceback (most recent call last):
  File "/Users/ldam/Documents/GIT/sohview/sohstationviewer2/sohstationviewer/view/main_window.py", line 891, in replot_loaded_data
    # resetting the is_stopping flag.
  File "/Users/ldam/Documents/GIT/sohview/sohstationviewer2/sohstationviewer/view/plotting/time_power_square/time_power_squared_dialog.py", line 268, in plot_channels
    widget = self.widget(i)
AttributeError: 'TimePowerSquaredDialog' object has no attribute 'widget'

• Tabs aren't removed all the way before creating new tabs.