DFNs API #262
Description
Introduce a DFN access API similar to the Models API (#134) to provide a smooth and consistent experience for acquiring/working with definition files.
We presently have an object model and related utilities for MF6 component definitions, including a TOML conversion facility and the beginnings of an approach to schema formalization/versioning (as refactoring of the existing DFN format/schema proceeds). Maybe worth using pydantic for this, if we do continue using it in other places (#260, cf modflowpy/pyphoenix-project#282).
There are not yet any automatic facilities for fetching DFN files from the MF6 repository, nor for juggling multiple versions of them, though we do have download tools to help with this. (Alternatively we could delegate fetching to Pooch as done for the Models API. Would be nice if we didn't have to maintain HTTP client code.)
Similar design considerations apply here as recently discussed in the Models API, re: loose coupling between devtools and remote resources it provides tooling to work with.
See also modflowpy/pyphoenix-project#246 for notes on scoping (separate layers of) the spec.