[feat] Add multiple argument process through the requires

[zeddo123]

This PR adds the possibility for a process to require the result of previous steps what are not directly before it.

let's say that a process (step 2) not only requires the result of the previous step but also the initial input to the workflow. With this PR, we can inject the needed results through the requires default argument when adding a step with the add method.

In this case the second step of the workflow requires the initial input of the workflow:

wf = Workflow('name').add(some_processing).add(lambda x y: x + y, requires='0')

The requires field expects the step numbers to be comma separated as such:

• 0,2 requires the result of step 0 (initial value) and step 2