Process functions: Allow nested output namespaces

[sphuber]

Up till now, the following was not possible:

@calcfunction
def add(x, y):
    return {'nested.output': x + y}

An exception would be raised by plumpy because the nested output namespace, defined by the ProcessSpec that is automatically generated from the function signature, does not contain the port output. The automatically generated process spec does mark the outputs namespace as dynamic, but this was not being applied recursively.

Not only is this functionality for users, the Parser.parse_from_node functonality is currently broken if the Parser returns output nodes in nested namespaces. The reason is that the parse_from_node creates a calcfunction on-the-fly which raises when it gets the outputs with the nested labels. Even though the original Process class may have specified these nested output namespaces, the on-the-fly calcfunction to capture the manual re-parsing does not support this.

The addition of this functionality requires a change in plumpy.

[unkcpz]

@sphuber thanks! I reviewed it yesterday, and since the test is passed, should be all fine.

[sphuber]

Thanks @unkcpz, I will just quickly add something to the documentation before merging this