Support more than one directory for PSyclone scripts

[hiker]

ATM there is no way to easily keep PSyclone optimisations script consistent across all sites. Links can't be used (fcm extract discards them), so what happens atm is that UKMO adds optimisation scripts to their directory, and no partner site gets this (unless they use a UKMO configuration for PSyclone).

Improve fab to support a hierarchy of directories for finding file-specific optimisation scripts, eg.:

1. Search for SITE/PATH/file.py
2. Search for default/PATH/file.py (maybe more directories? default, minimum??)
3. Search for SITE/global.py
4. Search for default/global.py

This would allow that optimisation scripts can be added and be used by all sites, but still each site would have the option to use their own custom script