Add Fab build script for gravity wave application

### Are there any linked Issues or Pull Requests?

# MetOffice/lfric_core#246, ticket: #240

### Brief description

This is the follow up from lfric_apps, #240 - adding a Fab based build system for gravity wave. It needs #240 (for an LFRic base class, and some other tools added).

### Further details of the issue.

This is just a command-line script, no integration into cylc.

Start by reviewing lfric_apps issue #240 and MetOffice/lfric_core#246 to understand the required LFRic base class and tools. Locate the gravity wave application's command-line entry point and define done as a Fab-based build script that builds the application without Cylc integration.

Written by the indexing model from the issue text.