Hotfix: Comment out X11 backend commands in the plotting example code

Newer versions of matplotlib require you to specify an X11 backend
library for matplotlib.pyplot.show() to work.  This requires us to
add an additional library to the gcpy_env environment. For now,
comment out the lines that invoke the X11 backend so as to allow
cloud benchmark simulations to work.  We will restore these once the
cloud container has been updated.

gcpy/examples/plotting/plot_comparisons.py
gcpy/examples/plotting/plot_single_panel.py
- Comment out the line to use the TkAgg backend.  This needs the "tk"
  package to be installed in the GCPy environment.

gcpy/docs/environment_files/environment.yml
- Add "tk" to the list of packages to install

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

docs/environment_files/environment.yml

@@ -35,6 +35,7 @@ dependencies:
     - scipy                           # Scientific python package
     - sparselt>=0.1.3                 # Regridding earth system model data
     - tabulate                        # Pretty-printing for column data
+    - tk                              # Tcl/Tk interface
     - xarray                          # Read data from netCDF etc files
     #
     # NOTE: These packages need to be pegged at specific versions

gcpy/examples/plotting/plot_comparisons.py

@@ -15,7 +15,7 @@
 import argparse
 import xarray as xr
 from matplotlib import use as mpl_use
-mpl_use("TkAgg")                          # X11 backend for plt.show()
+#mpl_use("TkAgg")                          # X11 backend for plt.show()
 import matplotlib.pyplot as plt
 from gcpy import plot
 from gcpy.constants import skip_these_vars

gcpy/examples/plotting/plot_single_panel.py

@@ -16,7 +16,7 @@
 import argparse
 import xarray as xr
 from matplotlib import use as mpl_use
-mpl_use("TkAgg")                        # X11 backend for plt.show()
+#mpl_use("TkAgg")                        # X11 backend for plt.show()
 import matplotlib.pyplot as plt
 from gcpy import plot
 from gcpy.util import rename_and_flip_gchp_rst_vars