Add create_soft_signal_r and creates_soft_signal_rw methods

[jsouter]

Moved from dodal, have placed in core.signals as seemed logical to prevent circular imports, could be moved to sim_signal_backends.py

[coretl]

Slight preference for:

Suggested change

	def create_soft_signal_rw(
	def soft_signal_rw(

[DominicOram]

Could: I would suggest splitting these into separate tests by either:

• Using pytest.mark.parametrize
• Have a helper function that is the test and call it with each case

[jsouter]

@gilesknap @coretl Add in option to pass an initial value to SimSignalBackend in the init method, and also supporting this in the soft_signal_* methods. self._initial_value gets set to the initial_value argument in SimSignalBackend's init, but the converter only converts it when connect is called, though I think this is fine if users are only accessing it through the public methods. soft_signal_r_and_backend still returns the backend, though this may not be required now.

[gilesknap]

@gilesknap @coretl Add in option to pass an initial value to SimSignalBackend in the init method, and also supporting this in the soft_signal_* methods. self._initial_value gets set to the initial_value argument in SimSignalBackend's init, but the converter only converts it when connect is called, though I think this is fine if users are only accessing it through the public methods. soft_signal_r_and_backend still returns the backend, though this may not be required now.

Thanks James, I have rebased and used the new feature successfully.

[jsouter]

Following discussion with @coretl, SimSignalBackends no longer take a source/name at init, and all backends now provide source as a method, where the Signal classes pass a name to the backend. source is available from signals as a property.
This shouldn't affect the CA/PVA backend much, but for devices using SimSignalBackend, the source descriptor entry is now based on the device's name, so the source may no longer resemble proper PV names, and will be more like "soft://my-sim-motor" instead of "soft://MY-SIM-MOTOR:X_RBV" e.g.

[evalott100]

Looks good!

[coretl]

Please can we make name=None be passed to these helpers, and if it isn't already there then add it as an optional arg to Signal.__init__

[coretl]

And also add this to epics_signal_* helpers

[jsouter]

Ah! Missed this comment somehow, might have to be a new PR.