Passthrough workflow type.

[riga]

This PR changes the way the --workflow TYPE parameter is passed between (workflow) tasks. Considering the following scenario,

graph TD
    A("TaskA (HTCondorWorkflow, LocalWorkflow)")
    B("TaskB (LocalWorkflow)")
    C("TaskC (HTCondorWorkflow, LocalWorkflow)")

    A --> B
    B --> C

running law run TaskC --workflow htcondor will currently

• (correctly) trigger TaskC to run on htcondor, before that
• (correctly) trigger TaskB to run locally, and before that
• (incorrectly?) trigger TaskA to run locally as well. Generally one would want to run TaskA on htcondor as well, but since TaskB is not able to do so, it will pass it's own workflow type, "local", upstream to TaskA.

In the current state, this could be avoided by configuring the requirements from TaskB to TaskA in a way that the task-level CLI parameter --TaskA-workflow would have preference, and then running law run TaskC --workflow htcondor --TaskA-workflow htcondor, but this is somewhat cumbersome and unexpected.

This PR stores the requested workflow type in a different parameter than the effective one and passes the former upstream (instead of the latter one). This is the new default behavior, which can be toggled back to the above description by setting passthrough_requested_workflow = False on task class level.