Update Fab to include LFRic_core and UM changes

[hiker]

A few small things:

• The compiler setup scripts have been renamed from script_gnu.py to setup_script_gnu.py. It would be useful if the setup in Fab would use the same naming convention.
• FabBase needs a name property (returning self._name)
• Add property for _site_config
• We need an option to add a root_symbol (not just setting it), or likely we need both :)
• Support setting of mirrors in a new update_repo callback (see UM). A more generic/better mirroring support might actually be useful. It would also involve to move the dependencies.yaml support class into Fab (seems to be the best place for a tool that will be used across various repos)
• Various updates to compiler setup scripts (documenting options etc)
• The pfunit tool (added in lfric_core Fix test warning #292) should likely be moved here, so testing can be used with more suites? Maybe even the pfunit_mixin?