XIOS handler restructure

[joewallwork]

XIOS handler restructure

Fixes #850
Fixes #1000
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.
• Better use of references to avoid unnecessary copies.
• Use of const qualifier in more places.

[joewallwork]

Okay I added some commits that address #1000, i.e., make better use of references (to avoid unnecessary copies) and use the const qualifier in all the places where arguments and loop variables should be const.

[Thanduriel]

Always happy to see more const!

The pass by value/ reference pendulum has swung a bit far in the other direction. For small arguments (e.g. fundamental types bool, int) it is generally best to pass them by value. Another area where value semantics are preferred is when assigning return values. There is a decent stack overflow post on the topic.

In a few places you could probably save a few lines by using structured bindings. However, this is just syntactic sugar.

[joewallwork]

Thanks @Thanduriel, these are all nice improvements! I've added them (plus a few extra similar ones) in my latest commits.