Use Tkinter in mesh tally plotter

This PR consists of an overhaul to plot_mesh_tally.py, the main purpose of which is to convert to Tkinter, the standard cross-platform GUI which is part of Python. The previous solution (PyQt) was fine, but introduced another dependency that was not needed, especially since the GUI is quite simple.

A few notes:
- The #! line has been changed from python2 to python. This reflects the fact that the plotter should work with either python 2 or 3.
- @nelsonag I've removed your while loop which allowed for a user to open a different file is the one they tried failed. This was mostly to separate the selection of a file from the GUI class itself. You'll see that the file is now selected in the `if __name__ == '__main__'` block. If the statepoint does not contain any mesh tallies, an error message is shown and the program exits.

I'm hoping you can review this @nelsonag since you had a previous PR with changes to this file.

This pull request closes #179.