Allow user defined product and task classes

[marr75]

I expected that I could define products and tasks of my own and then refer to them by their dotted path.

Unfortunately, they won't be validated or loaded. For example, I attempted to make a user defined version of "Link" that could be instantiated from a yaml spec and the error is:

ValueError: Error validating Task spec (class field): 'some_common_module.task_classes.Link' is not a valid Task class name

[edublancas]

@marr75 this makes sense. I see you are already working on a PR. feel free to request a review when ready!