Add workflow signature command option

Calling "balc --signature ..." provides a description of the workflow as
a nested workflow, with inputs and outputs specified. This is useful if
we add a requirement for nested workflows to specify their inputs and
outputs.  This is a planned modification, as it helps to verify that workflows
in different files remain compatible, and guide the user in the outer
workflow as to what inputs and outputs are available.
Require Nested Zapy workflows to use signature

Better documentation and checking of compatibility