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.
NF - When a user tries to perform an aggregate_by that tries to collapse an uncollapsible coord (e.g. a string based coord), then an informative error message is raised
Tweak message and add a test
Make tests optional that depend on pydot and pandas (optional installs).
Merge pull request #479 from pp-mo/options_tests

Make tests optional that depend on pydot and pandas (optional installs).
Merge pull request #504 from rhattersley/aggregate-tweak-rebase

Aggregate by on str dim err handling
Modified linear() to handle non-scalar length one coords.
Review actions
Fixed scalar issues.
propogate all coord names in construct midpoint
Merge pull request #508 from esc24/extrapolate_single_pt

Modified linear() to handle non-scalar length one coords.
Merge pull request #499 from esc24/coord_name_repl_bug

Propogate all coord names in construct midpoint
Load of nimrod files with multiple fields and period of interest

Signed-off-by: Andrew Munslow <munslowa@gmail.com>
Update post PR review

Signed-off-by: Andrew Munslow <munslowa@gmail.com>
Test result for period of interest test

Signed-off-by: Andrew Munslow <munslowa@gmail.com>
Use updated version of test data

old test files removed. extra newline added

test_osgb_to_latlon now uses the first index of the time dimension for the test

unused imports removed
Merge pull request #507 from munslowa/nimrod_time

Load of nimrod files with multiple fields and period of interest