Task pinning to workers

[gavento]

Add task attribute to allow only a subset of workers. An empty attribute

Usage:

• Working on files local to a worker (either reading or writing).
  • Useful for open and store tasks (but not limited to them).
• Selecting suitable worker subset for a taks when (virtual) resources are not appropriate.
  • However, for GPUs and other features, we should use resources.
• Testing.

Constant data objects already have a similar feature for placement, but this API is not planned to be published (and it is not really relevant for computed data objects).

Alternative: Every worker could be a resource, required by the task. However, this is not ergonomic and we do not have a clear picture of how to do (virtual) resources in the right way.

[gavento]

@spirali What is the status of this?

[spirali]

This is still implemented only for scheduler testing