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.