Save LAEA grid

[abooton]

Description
Saving of cubes on the Lambert Azimuthal Equal Area (LAEA) projection is added.
The LAEA template is described here: https://codes.ecmwf.int/grib/format/grib2/templates/3/140/

It is very similar to the Lambert Conformal projection specification in template_30, and so the coding was based on that.
https://codes.ecmwf.int/grib/format/grib2/templates/3/30/

Associated Issue
#344

[pp-mo]

I just realised that we have an outstanding previous request to do this (plus another thing) ... #295.
The discussion there seems to have stalled, but we should probably choose one over the other somehow.

[trexfeathers]

Thanks @abooton! I've compared this to the templates (thanks for the links), the Lambert Conformal code, and to #295; the changes make sense.

I have a few questions/suggestions below.

[trexfeathers]

@lbdreyer @abooton any thoughts on progressing this?

[trexfeathers]

Happy to close this if no longer important

[abooton]

Sorry - I get so much GitHub spam I had not realised it had review comments on it. This is still important. It may still be need operationally.

[trexfeathers]

Posting this outside any review comments to give me a common place to reference.

I tracked down template.3.140.def in eccodes, which I combined with ECMWF | Section 3 - Template 140 : Lambert azimuthal equal area projection to generate this:

Octets	Key	Type	Content
15	shapeOfTheEarth	codetable	Shape of the Earth (see Code table 3.2)
16	scaleFactorOfRadiusOfSphericalEarth	unsigned	Scale factor of radius of spherical Earth
17-20	scaledValueOfRadiusOfSphericalEarth	unsigned	Scaled value of radius of spherical Earth
21	scaleFactorOfMajorAxisOfOblateSpheroidEarth	unsigned	Scale factor of major axis of oblate spheroid Earth
22-25	scaledValueOfMajorAxisOfOblateSpheroidEarth	unsigned	Scaled value of major axis of oblate spheroid Earth
26	scaleFactorOfMinorAxisOfOblateSpheroidEarth	unsigned	Scale factor of minor axis of oblate spheroid Earth
27-30	scaledValueOfMinorAxisOfOblateSpheroidEarth	unsigned	Scaled value of minor axis of oblate spheroid Earth
31-34	numberOfPointsAlongXAxis	unsigned	Nx - numberOfPointsAlongXAxis
35-38	numberOfPointsAlongYAxis	unsigned	Ny - numberOfPointsAlongYAxis
39-42	latitudeOfFirstGridPoint	signed	La1 - latitudeOfFirstGridPoint
43-46	longitudeOfFirstGridPoint	signed	Lo1 - longitudeOfFirstGridPoint
47-50	standardParallelInMicrodegrees	signed	Standard Parallel In Microdegrees
51-54	centralLongitudeInMicrodegrees	signed	Central Longitude In Microdegrees
55	resolutionAndComponentFlags	codeflag	Resolution and component flags (see Flag table 3.3)
56-59	xDirectionGridLengthInMillimetres	unsigned	Dx - X-direction grid length in millimetres
60-63	yDirectionGridLengthInMillimetres	unsigned	Dy - Y-direction grid length in millimetres
64	scanningMode	codeflag	Scanning mode (see Flag table 3.4)

[abooton]

I have:

• Renamed standardParallel to standardParallelInMicrodegrees (in _save_rules.py and in the unit tests)
• Renamed centralLongitude to centralLongitudeInMicrodegrees (in _save_rules.py and in the unit tests)
• Added the raising the exception if the false_easting and false_northing are not zero. (I can see this can be configured when creating the LAEA coord-system, but there isn't the slot in the GRIB to save it, hence the need to raise the exception).
• I am happy that the lat and lon are set in degrees in the test _make_test_cube and are coded to be in micro-degrees in the test__template_specifics

[trexfeathers]

The docs build failure is not a concern for this pull request - #365

[trexfeathers]

Thanks very much for your contribution @abooton