Iris-grib

Bugfix release, mostly to address a performance problem with Dask 2 (190, cf. https://github.com/SciTools/iris/pull/3659)
Also ported grib-specific tests from Iris to reduce technical debt.
* 183 Fix typo in setup.py classifiers
* 184 Make testing compatible with latest Iris (2.3 / 2.4)
* 185 Get equalise_attributes to import from either Iris 2.4 or master (3.0+)
* 189 Get iris-grib tests passing with the latest version of eccodes (v2+)
* 190 Allow data-proxy objects to return zero-size slices without accessing the file, to stop it reading all field data during cube loads (Dask >= v2)
* 193 Pin eccodes to v1.9.3 in testing, to avoid problems with latest release
* Port across various grid-specific tests from Iris codebase
* 191 192 194 195 198 199 200 201 203

* Dropped Python 2 support (166, 170)
* Updated GRIB to CF metadata translations (143, 172)
* Support for loading GRIB messages with no fixed surface set (171)
* Support for saving cubes with a 'depth' coordinate (161)
* Support for calculating i/j increments if they are not set (148)
* Revert bug fix controlling hybrid pressure levels indexing (165)
* Addition of GRIB_PARAM attribute (156)
* Updates to setup.py (154, 180)
* Simplified file license headers (167)

For more details, please refer to the [iris-grib 0.15.0 release notes](https://iris-grib.readthedocs.io/en/latest/ref/release_notes.html#what-s-new-in-iris-grib-v0-15).

Added support for Python 3.

* Added WAFC aviation codes (129)
* Added PDT 4.15 (136)

Load and save data on hybrid vertical levels (fix existing hybrid pressure load).
* hybrid-pressure load 110; save 112
* hybrid-height load 103; save 102

Load and save Mercator projection(grid definition template 10).
* load 104; save 116

Save data on non-regular Lat-Lon grids (grid definition template 4).
* 113

Uses [ecCodes](https://software.ecmwf.int/wiki/display/ECC/ecCodes+Home) for the file interface.
This is ECMWF's replacement for the older [GRIB-API](https://software.ecmwf.int/wiki/display/GRIB/Home), which is now deprecated.