Provide utility function "file_is_newer_than" for results caching.
Simplify file load helper.
Added whats-new entry.  Slightly improved docstring.
Move 'tests.util.test_file_is_newer_than' into the unit-tests structure.
Tidied error case testing.
Merge pull request #787 from pp-mo/pickle_helper

Provide utility function "file_is_newer_than" for results caching.
MAINT: Refactored CubeList.merge

This change pulls out the two separate pieces from merge into private
method: one is for generating the protocubes; and the other is for
generating the resulting merged cubes from these protocubes.
ENH: Added capability to differentiate protocubes.

This adds functionality to the user to determine why a CubeList does not
result in a single cube.