Add CubeList.merge_cube().
Includes/builds-on contributions from bblay, pp-mo, and cpelley.
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().
Support lazy operations in cube.collapsed.