Refactor AdemData to fully fledged class.

[thclark]

The AdemData class is currently treated as a struct, with a group of functions operating on it.

It's also header-only, which makes for one very long file.

In this refactor:

• Make get_t2w a class method
• Make get_mean_speed a class method
• Make get_reylonds_stresses a class method
• Make get_spectra a class method
• Make function adem a class method, renaming it to something descriptive e.g. 'analyse'.
• Move function bodies out to adem.cpp

[thclark]

Removing from AURA project backlog as too much work already!