Configure UM via models

[penguian]

Closes #135

The changes are extensive.

1. The first main change is the introduction of the model variant, and a corresponding model/ directory. The string value of model determines which subdirectory of model/ to use to find the corresponding rose-app.conf file. There are currently 3 subdirectories, vn13 (default), vn13p0-rns (corresponding to the RNS suite u-dg768) and vn13p5-rns.
2. The second main change is that almost every environment variable in model/*/rose-app.conf now has a corresponding variant.
   1. The environment variables in rose-app.conf that take the values true or false correspond to either bool or off/on variants.
      1. The bool variants are normal Spack bool variants, taking the value False or True. The lambda functionbool_to_str transforms these to the environment variable values "false" or "true", determined by the value of the variant. The value given in rose-app.conf is ignored. Currently only eccodes and netcdf are bool variants. The default value of thesebool variables is True, corresponding to the value used in the RNS suite u-dg768.
      2. The off/on variants take the values "none", "off" or "on". The Spack default is "none". Leaving the value as "none" results in the corresponding environment variable being set to the value given in rose-app.conf. Otherwise, the lambda function off_on_to_str transforms "off" to "false" or "on" to "true" and this is the value used for the corresponding environment variable, overriding the value given in rose-app.conf.
   2. The component revision environment variables in rose-app.conf, other than um_rev, correspond to revision variants. For revision variants, the Spack default is "none". If the variant value left as "none", then if the value in rose-app.conf defined and non-empty, it is used as the environment variable value, otherwise the value is "um{version}" where version is the UM version. If the variant value is not "none" then it is used as the environment variable value, overriding any value in rose-app.conf.
   3. All other environment variables in rose-app.conf, with the exception of "um_rev", correspond to str variants. The default value for each of these variants is "none". Leaving this value as "none" results in the corresponding environment variable being set to the value given in rose-app.conf. If the variant is set to a value other than "none", then this value overrides the value given in rose-app.conf. This effectively makes the value in rose-app.conf the default.

[harshula]

Reviewed in VC.

• Put the defaults for conf variables in the *.conf files.
• Do not rename conf variables when used for variant names.
• Treat boolean variants as string variants by using strings "on"/"off"
• Add TODO comments where there is a good opportunity improve clarity by creating functions that can be called from multiple locations.
• Don't include DR_HOOK as a variant if it's not usable.

[penguian]

All comments addressed, except that eccodes and netcdf are boolean variants so that dependencies can be specified. These both default to True.

[harshula]

Looks good, well done!