Add Polar Stereographic CRS

[nhsavage]

✨ Feature Request

Allow Iris to correctly load data on a polar stereographic grid.

Motivation

We have acces to a high resolution digital terrain model of the south pole which we wish to convert to the UM input files for orography. We cannot use this data at the moment, as ANTS relies on Iris being able to load the data. Regridding to say a lat-lon grid is not appropriate as that either has very large numbers of points close together at the pole or we lose a lot of information at lower latitudes

Additional context

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Sample output from ncdump

netcdf REMA_1km_dem_filled {
dimensions:
	x = 5500 ;
	y = 4500 ;
variables:
	char polar_stereographic ;
		polar_stereographic:grid_mapping_name = "polar_stereographic" ;
		polar_stereographic:straight_vertical_longitude_from_pole = 0. ;
		polar_stereographic:false_easting = 0. ;
		polar_stereographic:false_northing = 0. ;
		polar_stereographic:latitude_of_projection_origin = -90. ;
		polar_stereographic:standard_parallel = -71. ;
		polar_stereographic:long_name = "CRS definition" ;
		polar_stereographic:longitude_of_prime_meridian = 0. ;
		polar_stereographic:semi_major_axis = 6378137. ;
		polar_stereographic:inverse_flattening = 298.257223563 ;
		polar_stereographic:spatial_ref = "PROJCS[\"unnamed\",GEOGCS[\"WGS 84\",DATUM[\"WGS_1984\",SPHEROID[\"WGS 84\",6378137,298.257223563,AUTHORITY[\"EPSG\",\"7030\"]],AUTHORITY[\"EPSG\",\"6326\"]],PRIMEM[\"Greenwich\",0],UNIT[\"degree\",0.0174532925199433],AUTHORITY[\"EPSG\",\"4326\"]],PROJECTION[\"Polar_Stereographic\"],PARAMETER[\"latitude_of_origin\",-71],PARAMETER[\"central_meridian\",0],PARAMETER[\"scale_factor\",1],PARAMETER[\"false_easting\",0],PARAMETER[\"false_northing\",0],UNIT[\"metre\",1,AUTHORITY[\"EPSG\",\"9001\"]]]" ;
		polar_stereographic:GeoTransform = "-2700000 1000 0 2300000 0 -1000 " ;
	double x(x) ;
		x:standard_name = "projection_x_coordinate" ;
		x:long_name = "x coordinate of projection" ;
		x:units = "m" ;
	double y(y) ;
		y:standard_name = "projection_y_coordinate" ;
		y:long_name = "y coordinate of projection" ;
		y:units = "m" ;
	float Band1(y, x) ;
		Band1:long_name = "GDAL Band Number 1" ;
		Band1:_FillValue = -9999.f ;
		Band1:grid_mapping = "polar_stereographic" ;