Add note on installing tkinter for optview (#453)

Author: A-CGray

doc/install.rst

@@ -93,6 +93,7 @@ To install pyOptSparse including those dependencies, run
 
     pip install .[optview]
 
+If you encounter a ``no module named tkinter`` error when trying to run optview, you may need to `install tkinter on your system <https://stackoverflow.com/questions/25905540/importerror-no-module-named-tkinter>`__. 
 
 Testing
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