Allow constants in kwargs of derived_signal

[shihab-dls]

Fixes #857

This PR should allow literals of type int | bool | str | float to be passed in as raw_devices. The approach that I've taken is filtering out these constants in SignalTransformer, then passing them back in when a derived reading is made. There are probably more streamlined ways of doing this, so this is an initial pass to check that this is the intended functionality.

[DominicOram]

The typing is all a bit messy but I'm not sure if there's a better way to do it. Functionally works great, just the one comment. Also, could you add something to https://blueskyproject.io/ophyd-async/main/how-to/derive-one-signal-from-others.html?

[coretl]

Neat! A couple of suggestions to tidy...

[shihab-dls]

If we have a multi derived signals using transforms, we still need to pass a signal as a parameter, as this will be a transform_device. Splitting into raw_constants, raw_devices, transform_constants, and transform_devices lets us unpack raw_constants when reading raw_values, and unpack transform_constants when populating the transform with args. This should thus be general for passing constants in single and multi derived signals. So the order of splitting this up is now:

• Take raw_and_transform_devices_and_constants in DerivedSignalFactory
• Split into _raw_and_transform_devices and _raw_and_transform_constants for checking against transform_cls, as this lets us type(v) for constants and v.datatype or get_locatable_type(v) for devices.
• Split into raw_constants, raw_devices, transform_constants, and transform_devices before passing to SignalTransformer

Then when we collect our derived readings we can:

raw_values = {
    **{
        k: raw_and_transform_readings[sig.name]["value"]
        for k, sig in self._raw_devices.items()
    },
    **self._raw_constants,
}

and

return self._transform_cls(**(transform_args | self._transform_constants))

in _make_transform_from_readings. So, given a single derived signal, a constant would be populated as a raw_constant, and no transform_constant is passed as no specific transform is given. And, given a multi derived signal, a constant would be populated as a transform_constant and no raw_constant is expected (i.e., parameters should be passed as type hints to the transform, and thus a transform_constant).