Change MetadataLike.id to return an Identifier

[dlindhol]

There are currently places where we use string equality for DataType (a.k.a. "variable") identifiers (e.g. DataType.findVariable). Presumably we can do this via a new method DataType.hasName(name: String): Boolean.

We should be mindful of a potential use case of supporting aliases via the same hasName method (#58 ). Is there risk of being too loose with alias matching when a primary identifier match is needed? Note that this is the motivation for getVariable vs. findVariable. The former is designed to match identifiers only.

It's also worth considering whether the id method should continue to come from MetadataLike (considered experimental) or if it should belong to DataType. The only other implementor is Dataset. Is there value in factoring the concept into MetadataLike? Maybe another name? An implicit class?