documentation to explain tasks better

[jaberg]

After discussing with @npinto Jan 31 it became clear that he had mis-understood my intention with tasks. He suggested renaming them to reflect the fact that "Tasks" are not new datastructures so much as tuples of lists of things with specific formats. Contrast "meta" which has almost no formatting guideline.

It might be useful to have a documentation example that says how to deal with a learning problem that doesn't fit neatly into one of the provided tasks.