Hide new API.

Change from:
    iris.fileformats.grib.GribMessage
to:
    iris.fileformats.grib.message._GribMessage.
Use both _GribMessage() and _RawGribMessage().
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.
Allow users to choose the number of processors used for running tests.