Add child protocol

[rosesyrett]

It has become useful in ophyd.v2 to add a children property to devices.

To remain consistent with the parent property, it was suggested that adding a protocol like HasChildren, analogous to HasParent, would be useful

Description

A PR has been started about this for ophyd.v2. See here: bluesky/ophyd#1138

How Has This Been Tested?

I am unsure how protocols are tested, please advise and I will add tests.

[rosesyrett]

not sure why lint is failing, seems to be complaining about something my changes have not altered.

[untzag]

@RAYemelyanova consider adding some tests

bluesky/tests/test_protocols.py

Also can this be an @property so that instance attributes are valid?

[untzag]

For what it's worth this isn't the implementation I happened to go with for yaqc-bluesky, I just have a list of child objects. I don't know how important the concept of a device name is to bluesky, so I don't really know why we want tuples of (name, device).

That said I'm all for standards and my devices do have names so I'd be happy to change yaqc-bluesky to match this protocol once merged.

We could look at e.g. instrbuilder to see about other implementations.