Add support for retrieve high resolution exports from WCS

[rowanwins]

Currently WCS exports are done based on the width and height of the current map (eg 1000px by 600px )and resulting images don't represent the full resolution.

Instead of passing width and height parameters to the GetCoverage request we can send resx and resy specifying the required resolution, this is sent in decimal degrees.

https://ows.dea.ga.gov.au/?service=WCS
&request=GetCoverage
&version=1.0.0
&format=GeoTIFF
&crs=EPSG%3A4326
&resx=0.0001&resy=0.0001
&coverage=s2_ard_granule_nbar_t
&bbox=121.89074847916314,-17.99554269281186,122.57921195492706,-17.629104173950257&time=2019-10-06T00%3A00%3A00.000Z&styles=infrared_green

Alternatively you can swap things across to Web Mercator

https://ows.dea.ga.gov.au/?service=WCS
&request=GetCoverage
&version=1.0.0
&format=GeoTIFF
&crs=EPSG:3857      <---- set me to web mercator
&resx=10&resy=10    <---- set me a sensible resolution
&coverage=s2_ard_granule_nbar_t
&bbox=13568816.053,-2037026.831,13645455.457,-1994181.130 <---- set bbox to web mercator
&time=2019-10-06T00%3A00%3A00.000Z
&styles=infrared_green

I think the later is preferable

[rowanwins]

Also noted here https://github.com/TerriaJS/terriajs/issues/3109#issuecomment-427740526

[nf-s]

No longer relevant