Make .parent a read-only property

[coretl]

At the moment this is a read/write attribute on the class set from the outside. It is only ever set by its parent, so could be passed in the creation of children like in ophyd, but don't want to make the epics_signal_rw case messier:

self.x = epics_signal_rw(int, prefix+"x", parent=self)

There's not a nice way round setting the parent apart from doing child._parent = self # noqa: SLF001 in the __setattr__ method.

We should discuss

Acceptance Criteria

• .parent is a read-only public property with a suitable mechanism for setting

[evalott100]

We should also use name mangling so parent isn't overwritten by subclasses

#594 (comment)