Refactor seasons categorisation functions.

Deprecates the add_custom_season* functions in favour of adding their
functionality to the existing season categorisation functions via a
keyword argument.
Removal of strict constraint in izip().
Merge pull request #490 from ajdawson/refactor-seasons-categorisation

Refactor seasons categorisation functions.
Review actions.
Merge pull request #495 from esc24/izip_extra_aux_coords

Removal of strict constraint in izip().
PP save with no time forecast.
Merge pull request #498 from bjlittle/pp_save_time

PP save with no time forecast.
More descriptive error when aggregate_by() tries to collapse an uncollapsible coord (e.g. a string based coord)