Auto-generate zero data for missing data payload

[bjlittle]

This pull-request extends the capability of the iris_grib.message._DataProxy to detect and auto-generate missing data payload for a GRIB2 message.

The scenario is that the message data payload is all zero, and rather than serialize this within the message, thus increasing the footprint, an application can generate the data at runtime whenever the Data Representation Section [5] has bitsPerValue=0 and an empty Data Section [7].

Failure to support this extension results in no encoded Data Section [7] within a iris_grib.message._RawGribMessage causing a KeyError traceback within the user script. The impact of this is that an N-D Iris cube can be loaded but whenever at least one GRIB2 message has such missing data, touching the cube data will result in this failure.

Reference: SciTools/iris-test-data#89

[bjlittle]

@pp-mo As a result of this fix, we now have "unexpected successes" in the tests regarding pickling.

We could now simply not mark these tests are expected failures.

How would you like to proceed?

[bjlittle]

Out of curiosity, I remove the @unittest.expectedFailure decorators ...

[pp-mo]

We could now simply not mark these tests are expected failures.

After some chat, I think this is OK.
I'll do some back-fill to check what the error USED to be, but I suspect we just found that latest-eccodes has fixed it (= made the pickling possible)

[pp-mo]

Unless you want to follow a tiny suggestion..
good to go I think

[bjlittle]

@pp-mo Sweet, cheers 💯