Resolve iris-grib test failures

[bjlittle]

See #3446, in particular this comment.

Results in the following errors=10, failures=5 for iris-grib tests, see this associated travis-ci log.

[pp-mo]

A couple of related issues that this ought to close :

• Get iris-grib testing working #3375
• iris_grib failure #3070

[bjlittle]

This is also related:

• TEST: Extends #2569 to unpickle #2608

[lbdreyer]

FYI:
When getting iris_grib (and python-eccodes) to correctly install in the travis job, you can use the installation in the iris-grib repo as a reference. But note that python-eccodes is now available from conda-forge, so we should install it from there rather than a pip install as is done in iris-grib

[bjlittle]

When installing from conda-forge note the approach used in travis-ci, which ensures that installing iris-grib doesn't install iris as it's dependency - which could replace the development version of iris being tested.

[stephenworsley]

I have begun to adress these in #3469.

[lbdreyer]

#3469 has addressed most of the test failures, skipping the three that cannot be fixed in iris.

I have raised an new PR to target the work of moving these tests to iris-grib where the necessary fixes can be applied