Introduce tasks which produce intermediate results

[WardLT]

Overhaul how tasks are defined in Colmena to provide the ability to distinguish tasks which produce intermediate results from those that do not.

Previously, we wrapped functions being invoked by a workflow engine in a function which handled the Colmena-specific things like unpacking results.

The new approach is simply a class-based strategy for wrapping. The logic which was in the wrapper function is now in the __call__ method of the class, and the function being invoked is a attribute of the class.

The class based approach gives more opportunities for creating subclasses, like the new PythonGeneratorTask and the legacy ExecutableTask class, which invoke the user-provided function differently.

To do list:

• Write unit tests for new models
• Update task servers to use new models
• Add a new section to the documentation about task types
• Unbreak the MPI example