XIOS: Prune the handler class

[joewallwork]

While working on integrating XIOS, we provided interfaces to a large subset of XIOS' overall functionality to cover everything that we thought we might need. As the work developed, it became apparent that some of the functions we ported either aren't required or are only ever set to one value. As such, it would make sense to go through the interface and:

• Drop any methods that are never used.
• Privatise any methods that shouldn't be customisable by the user.
• Inline any methods that are only used internally and only at one call site.

[joewallwork]

This can be done at the end of the project to tidy up.