Custom function extension #771

Implement a helper Extension that makes it easy to create a custom Extension that takes no manager as an argument and executes a pure custom function controlled by a trigger.

Custom functions that do not take manager as an argument can perform two main operations

• Side-effect operations on arguments
• report on execution ppe.reporting.report()

As for side-effect operations, it is not possible to control them as a function of PPE due to the python specification, so no specific action is taken.

The report function would be more convenient if it could report on values returned by dict, following the PPE's handler module.

Therefore, I would implement it with the following interface

class UserFunctionExtension(Extension):
    priority = PRIORITY_WRITER
    def __init__(self, fn: Callable[..., Dict[str, Scalar]], args: Sequenc[Any], kwargs: Mapping[Any], report_prefix: str = None, priority: int = PRIORITY_WRITER) -> None:
        ...