Hide new API.

Change from:
    iris.fileformats.grib.GribMessage
to:
    iris.fileformats.grib.message._GribMessage.
Use both _GribMessage() and _RawGribMessage().
Added likely location of test images to git ignore file.
Merge pull request #1314 from ajdawson/image-output-location

Added likely location of test images to git ignore file.
Watch out for scanning mode
Merge pull request #1313 from rhattersley/grib-data

Fix new GRIB data interpretation
Switch to simultaneous conda install.
Merge pull request #1316 from rhattersley/test-fix

Switch to simultaneous conda install.
New GRIB2 load convert mechanism
Raise TranslationError exceptions + doc-string
Initial unit mock tests
Correction to typo
Unit test scanning mode.
Rebase + coding standards