Support explicit one-way dependencies

[aplowman]

Consider two tasks: task_1 and task_2 that each have an input parameter: parameter_A. Assuming no extra tasks generate parameter_A as an output, a value of parameter_A can be given once in either of the tasks, and it will be used to parametrise both tasks. However, consider an additional constraint: task_1 generates another parameter: parameter_B, which is then required by task_2. In this case, the task dependency must be: task_1 --> task_2. Thus, parameter_A can be specified only in task_A now, since otherwise the workflow would be circularly dependent.

However, there are scenarios where we would want to specify parameter_A only on task_2. For instance, if the parameter is optional for task_1, but required for task_2. To satisfy this requirement, we should support the ability to exclude particular tasks from the dependency resolution of a given parameter. E.g., we could say that task_1 should not get its parameter_A value from task_2, despite our specification of parameter_A only within task_2.

Due to: LightForm-group/UoM-CSF-matflow#14