Convenience helpers for interacting with `DataModel`s

An implementation of DataModel that stores the data in working memory itself.

Supports using `src` in data model for CLJ ONLY, which must be a URI that clojure.java.io/reader
would accept.

Data retrieval resolves scopes by walking up the tree looking for parent states that have the data desired.

`current-data` is a merge of all state from root to the contextual state.
`get-at` will NOT walk scopes, but supports simple keywords for the current context, and paths of the form [state-id data-key].