Make StandardReadable _read_signals, _configuration_signals and _read_uncached_signals publically accessible

[callumforrester]

Required as short-term solution for #192, these attributes should be accessible from other devices. I suggest using a read-only property e.g.

class StandardReadable:
    ...
    @property
    def read_signals(self) -> List[SignalR]:
        return self._read_signals

Ditto for the others.

Acceptance Critera

• Can call .read_signals, .configuration_signals and .read_uncached_signals on a StandardReadable
• Tests to prove this/prevent regression

[coretl]

I would prefer to make the decision on #192 first, as we may not require it if we choose a different route there...

[coretl]

Closed by #240