Introduce HasChannel

[liamhuber]

A new mix-in class that demands that children have a channel -> Channel property.

The Channel class is re-parented onto this and returns self for the property. This forms a tautology, but doesn't really hurt anything.

Now, however, when the IO classes are transforming assignments into connections, all they need to do is check if the assignee is a HasChannel instance and use its channel. In our case, SingleValueNode(FastNode) -> SingleValueNode(FastNode, HasChannel), where channel is just the single output channel. This simplifies the stuff in IO and is re-usable in case we have other classes we want to be able to (attempt!) to connect to (the connection validity will still get tested during the connection).

[coveralls]

Pull Request Test Coverage Report for Build 4855215939

• 23 of 25 (92.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 12.867%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyiron_contrib/workflow/has_channel.py	8	10	80.0%

Totals
Change from base Build 4855037198:	0.1%
Covered Lines:	1580
Relevant Lines:	12279

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💛 - Coveralls