Automate retrieval of CMIP data with managecmip

As discussed during refinement of #130, users are currently expected to either have CMIP data already on disk, or to retrieve the data using managecmip manually themselves. This ticket covers the creation of CMEW app that automatically retrieves CMIP data with managecmip should it not already exist on disk.

Start by reading the refinement discussion in #130 and reviewing how users currently retrieve CMIP data manually with managecmip. Define the CMEW entry point and integration scope before implementing; the work is done when missing CMIP data is retrieved automatically and existing on-disk data continues to work.

Written by the indexing model from the issue text.