Add CubeList.merge_cube().
Includes/builds-on contributions from bblay, pp-mo, and cpelley.
Merge pull request #921 from esc24/hybridpressure_pp

Hybrid Pressure load and save support for PP
MergeError docstring tidy
Review tweaks
Add tests for error_on_merge permutations.
Move _CoordSignature.match_payload to _CoordPayload.match_signature
Make DuplicateDataError subclass MergeError
Merge pull request #991 from rhattersley/mergefail

Add CubeList.merge_cube().
Fixed issue of cross year end datetime-like cell comparison