Fix multi-message handling.
Merge pull request #1317 from rhattersley/multi-message

Fix multi-message handling for new GRIB loader.
New GRIB2 load convert mechanism
Raise TranslationError exceptions + doc-string
Initial unit mock tests
Correction to typo
Unit test scanning mode.
Rebase + coding standards
Review actions
Make is_circular utility function private
Fix for no UM to CF translation