Resolution keyword automatically added

[uniomni]

Currently, the native resolution is stored in an extra keyword. This loosens the one-to-one relationship we had between keywords in geoserver and on file. It works but it is slightly annoying. Also the resolution value has to be a string so it needs to be converted back to float before using.

It would perhaps be better to leave the keyword as strictly user-defined and instead store the OWSLib metadata dictionary with
layer instances. Then all metadata would be available in any format. Not sure how important or doable this is, but now it has been flagged.

Associated with this issue, I can't decide whether to keep resolution as a 2-tuple (resx, resy) as gdal and geoserver uses or to go with a single cellsize which is what is actually stored in e.g. asc files and what is needed for scaling. Unfortunately, resx and resy are sometimes noticably different. This is now issue #173

[uniomni]

Mainly done on 28th October in changeset ec0d729#diff-0

[uniomni]

Closing this for now with the option of reopening if we can store the native resolution differently one day....