Add a URI subclass of Scalar

[dlindhol]

We currently use regular text variables to represent URIs. In latis2, we use naming conventions ("file" or "url") to pick out the URI variables. This is limiting and less safe.

With a Uri class that extends Scalar (invoked with a class property in the fdml), we could more safely pick out the URI variable (e.g. in a granule list dataset).

Presumably we would encapsulate some URI utility methods here, though we need to be mindful of other URI use cases (e.g. reading datasets). Maybe make a uri package to encapsulate all things URI? This is akin to Time.

Another question to consider: Should we continue to use the java.net.URI class internally? Or http4s? Or...?