Simplify the named tuple / dataframe conversion stuff

[drbenvincent]

This is made complex at the moment because of the nested named tuple I am using for the DARC design space. If I just use a single named tuple with names for each of the design dimensions then we should be able to use automatic tuple/dataframe conversion functions. This should mean less code, and less that a toolbox developer has to write.

Will require the psychopy DARC demos to be updated.