Use threading or other approach to keep GUI from freezing

[bmcrae]

Right now the long-running solves mean that the GUI freezes up and also that it can get buried under other windows (at least when running on MS Windows). Not sure if threading (e.g. as discussed in link below) can help:
http://www.blog.pythonlibrary.org/2010/05/22/wxpython-and-threads/

This can wait for HTML gui if needed.

We may want to set default screenprint_log = True since messages sent to the console appear more fluid and real-time (and the console window doesn't get buried).