Add panda-specific save load plans

[olliesilvester]

Adds plans to save and load a panda device, including loading the phases in the correct order. This uses the functions added in #36

This also fixes an issue with hanging subprocceses in unit tests, and fixes a bug where devices wouldn't load if one of the saved PV's were ignored (set to None in the yaml file)

[coretl]

I already wrote this yesterday for i22, so I'll paste in what I had in case you want to use any of it:

def save_the_panda():
    signals = walk_rw_signals(panda)
    values = yield from get_signal_values(signals, ignore=["seq1.table"])
    units = {n: values.pop(n) for n in list(values) if n.endswith("units")}
    save_to_yaml([units, values], "/tmp/thing.yaml")

[coretl]

Thinking about this, the load is completely generic, it is not specifically for PandA, so I think we should remove it from here...

[rosesyrett]

I'm not sure I like the fact we have plans in ophyd-async. I think this needs a broader discussion...

In general I think this looks good if we are heading in that direction, but personally I'd prefer if we could move this into dls-bluesky-core and keep ophyd-async bluesky agnostic where possible...

[rosesyrett]

Gonna improve the test coverage and fix the conflict with main

[rosesyrett]

@coretl and @olliesilvester I've just been adding a test case for this:

def set_signal_values(
    signals: Dict[str, SignalRW[Any]], values: Sequence[Dict[str, Any]]
) -> Generator[Msg, None, None]:
    """Maps signals from a yaml file into device signals.

    ``values`` contains signal values in phases, which are loaded in sequentially
    into the provided signals, to ensure signals are set in the correct order.

    Parameters
    ----------
    signals : Dict[str, SignalRW[Any]]
        Dictionary of named signals to be updated if value found in values argument.
        Can be the output of :func:`walk_rw_signals()` for a device.

    values : Sequence[Dict[str, Any]]
        List of dictionaries of signal name and value pairs, if a signal matches
        the name of one in the signals argument, sets the signal to that value.
        The groups of signals are loaded in their list order.
        Can be the output of :func:`load_from_yaml()` for a yaml file.

    See Also
    --------
    :func:`ophyd_async.core.load_from_yaml`
    :func:`ophyd_async.core.walk_rw_signals`
    """
    # For each phase, set all the signals,
    # load them to the correct value and wait for the load to complete
    for phase_number, phase in enumerate(values):
        # Key is signal name
        for key, value in phase.items():
            # Skip ignored values
            if value is None:
                continue

            if key in signals:
                yield from abs_set(
                    signals[key], value, group=f"load-phase{phase_number}"
                )

        yield from wait(f"load-phase{phase_number}")

Specifically, for the 'continue' line. I've just noticed; isn't the purpose of this, so that yaml files with 'null' entries are ignored? If so, why not just remove that line from the yaml file, especially considering the confusion this can cause with enums that have 'None' values? Edit: on the other side of this, we can ensure that when we are saving to yaml, we don't save any pv's that have 'None' values outside of enums.

[coretl]

Specifically, for the 'continue' line. I've just noticed; isn't the purpose of this, so that yaml files with 'null' entries are ignored? If so, why not just remove that line from the yaml file, especially considering the confusion this can cause with enums that have 'None' values? Edit: on the other side of this, we can ensure that when we are saving to yaml, we don't save any pv's that have 'None' values outside of enums.

If the value is None, this means the signal was present on the Device, but ignored. This means you can tell the different between a new signal being added to a Device (which should be saved if you re-save), and an ignored Signal

The continue looks correct to me, if you see a null in the file (which will be translated to a None in the python dict), then don't set that value

[rosesyrett]

Just rebased