Write GRIB1 loading tests to modern standards

[trexfeathers]

Iris-grib has been loading GRIB1 for years, so to the best of our knowledge it is loading things correctly in our users' eyes. Write a series of tests that thoroughly assert the current behaviour as the desired behaviour, by examining the loaded Cubes. This is the principle as Assert-CML, if we wanted to go that direction.

This work is required to support #56 - we can make sweeping refactors and know that we have not changed behaviour.

Files to check

$ cd ...git/iris-test-data/test_data/GRIB
$ find . -iname "*.grib1"
./lambert/lambert.grib1
./rotated_uk/uk_wrongparam.grib1
./reduced/reduced_ll_missing.grib1
./reduced/reduced_ll.grib1
./shape_of_earth/global.grib1
./time_processed/time_bound.grib1
./gaussian/regular_gg.grib1

Possible implementations

• AssertCML
• Cube equality against saved NetCDF Cubes
• Many specific asserts