Implemented "constrained" liner spectral unmixing example.
Edited the mix function to create a pixel made of two materials.

Modified the mix function to create a pixel that is either constrained or unconstrained in it's mixture.
Edited the get_image function.

Changed the function round the values that it was creating for the pixel_values.
Edited the sensordesign.py file.

Inside of the test_data added the constrained= false argument.
Merge pull request #1 from wc351/development

Development
Initial implementation of Scenario system.

Implemented a scenario system to allow for multiple scenarios to be launched from a text based menu.  Scenario system required the following changes:

1) Updating materials data to be more clearly named (ex: simple_materials to two_materials)
2) Creation of scenario class that requires a name and materials.  sensordesign.py contains the text based menu.  Scenarios are created in scenarios.py as Scenario objects in the scenarii list which is imported into sensordesign.py.  Portions of original sensordesign.py ended up in the Scenario class and some portions, such as the band creation menu, ended up in sensing.py.
3) Corrected the create_grid function in sensing.py to properly handle data involving mixed pixels.
4) Moved the band creation menu system that was housed in sensordesign.py into sensing.py because all scenarios will to make sensordesign.py more manageable.
Update readme with todo.
Improved readme GitHub markdown.
Corrected embarrassing misspelling in readme.
sense function takes Band named tuple instead of start, stop lambda

 Updated sense function to take a Band named tuple instead of a start_lambda and stop_lambda.  Updated all calls to sense to properly pass a band tuple.
One more README brush up.