GUI: Set default save format to .pdf (#243)

Empress GUI now defaults to saving images as .pdf instead of .png. The pdf format is more helpful because it's a vector format that can be zoomed in infinitely.

empress_gui.py

@@ -9,11 +9,12 @@
 import sys
 import pathlib
 
+import matplotlib
+matplotlib.rcParams['savefig.format'] = 'pdf'  # default save format
 from matplotlib import pyplot as plt
 from matplotlib.backends.backend_tkagg import FigureCanvasTkAgg, NavigationToolbar2Tk
 
 import empress
-from empress import input_reader
 from empress.recon_vis.utils import dict_to_tree
 from empress.recon_vis import tree