XIOS handler restructure

[jwallwork23]

XIOS handler restructure

Fixes #850
Merges into #974

This will hopefully be the last major restructuring of the XIOS handler class.

The PR includes some pruning:

• Remove unused enableXios function.
• Drop errors from the ParaGridIO_Xios member functions already thrown in Xios.read and Xios.write.
• Cut down the configGetX member functions.
• Remove the contextId and calendarType arguments from the constructor because they're now set in iodef.xml as of Move calendar, axis, and grid config to XML #993.

It also includes better modularisation:

• Extract a new setupFiles member function out of the constructor.
• Rename parseInputFiles as setupFields.
• Split the configuration into setupClient, setupContext, and setupCalendar and have all of them called from configure alongside setupFiles and setupFields.

Other misc changes:

• Additional checking of the calendar setup.
• Reordering of member functions to better group XIOS concepts together.