Workflow parameters

[riga]

This PR adds workflow parameters, a way to parametrize workflows in a way that makes the branch lookup dynamic, based on values in the branch map.

From the README:

---

Example code:

import law
import luigi


class MyWorkflow(law.LocalWorkflow):

    option_a = law.WorkflowParameter()
    option_b = law.WorkflowParameter(cls=luigi.IntParameter)

    @classmethod
    def create_branch_map(cls, params):
        return {
            0: {"option_a": "foo", "option_b": 123},
            1: {"option_a": "bar", "option_b": 456},
            2: {"option_a": "foo", "option_b": 456},
            # ... you could add more branches here
        }

    def run(self):
        # this run method is only called by *branches*, i.e.,
        #   - self.branch will be 0, 1 or 2, and
        #   - self.branch_data will refer to the corresponding dictionary in the branch map

        pass  # ... implementation

option_a and option_b are law.WorkflowParameters that can optionally define a cls (or inst) of a other parameter object that is used for parameter parsing and serialization (as always).

create_branch_map becomes a @classmethod and receives all task parameters in a dictionary params.
This is necessary since the automatic lookup of branches based on the values of workflow parameters, internally, must happen before the task is actually instantiated.

As a result, parameters can be defined verbosely on the command line, translate to branch values, and configure which branches are run by the workflow:

• --option-a foo --option-b 123 → finds branch=0 (but for this, you probably wouldn't need a workflow in the first place)
• --option-a foo → finds branches=[0, 2] and runs them as a workflow
• --option-b 456 → finds branches=[1, 2] and runs them as a workflow
• --option-b 123 → finds branches=[0] and runs it in a workflow; in particular, this is not a branch as --option-a is not specified, meaning we're "not interested in any particular value of option_a, just run all that are found".

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Closes #131, #137.