Fix images in MPB data analysis tutorial (NanoComp#695)

* Fix images in MPB data analysis tutorial

* Add matplotlib troubleshooting for mac

* Import meep before matplotlib to work around macOS issues

* Fix typo in docs

doc/docs/Installation.md

@@ -39,7 +39,7 @@ This creates an environment called "mp" (you can name this anything you like) wi
 Next, we need to activate the environment before we can start using it.
 
 ```bash
-source activate mp
+conda activate mp
 ```
 
 Now, `python -c 'import meep'` should work, and you can try running some of the examples in the `meep/python/examples` directory.
@@ -50,7 +50,7 @@ Installing parallel PyMeep follows the same pattern, but the package is called `
 
 ```bash
 conda create -n pmp -c chogan -c conda-forge pymeep-parallel
-source activate pmp
+conda activate pmp
 ```
 
 The environment includes `mpi4py`, so you can run an MPI job with 4 processes like this:
@@ -61,6 +61,11 @@ mpirun -np 4 python <script_name>.py
 
 If you run into issues, make sure your `PYTHONPATH` environment variable is unset.
 
+**Note:** If you experience crashes when using `matplotlib` on macOS, try importing `meep` before importing `matplotlib`. In addition add the following line to your `~/.matplotlib/matplotlibrc` file to force the `TkAgg` backend:
+```
+backend: TkAgg
+```
+
 **Note:** For pymeep-parallel on macOS, a [bug](https://github.com/open-mpi/ompi/issues/2956) in openmpi requires that the environment variable `TMPDIR` be set to a short path like `/tmp`. Without this workaround, you may see errors similar to this:
 
 ```bash

python/examples/mpb_data_analysis.py

@@ -2,20 +2,19 @@
 
 import os
 import sys
-import matplotlib.pyplot as plt
 import numpy as np
 import meep as mp
+import matplotlib.pyplot as plt
 from meep import mpb
 
 examples_dir = os.path.realpath(os.path.dirname(__file__))
 sys.path.insert(0, examples_dir)
 
-# Import the ModeSolver from the mpb_tri_rods.py example
-from mpb_tri_rods import ms as tr_ms
-from mpb_diamond import ms as d_ms
-
 
 def tri_rods():
+    # Import the ModeSolver defined in the mpb_tri_rods.py example
+    from mpb_tri_rods import ms as tr_ms
+
     efields = []
 
     # Band function to collect the efields
@@ -57,6 +56,9 @@ def get_efields(tr_ms, band):
 
 
 def diamond():
+    # Import the ModeSolver from the mpb_diamond.py example
+    from mpb_diamond import ms as d_ms
+
     dpwr = []
 
     def get_dpwr(ms, band):