added subplot convenience functions

[sanketdiwale]

Summary of changes:

1. Used matplotlib.gridpec to create a gridspec object that keeps track of the desired layout. Can call a layout creation by calling the method make_subplots with arguments passable to gridspec.

2. moved set_font call in add_plot to be above the axes function calls as with subplots, the font size was not correctly set if the call was made after making the axes function calls.

3. added a set_subplot method to allow user to select the subplot axis on which to add_plot

4. added example for usage in the subplots section of the jupyter notebook

5. added a line plt.ion to automatically turn on interactive mode if it is not already on as even the basic plotting was not working without this mode on