```GDAL_NANSAT_``` in netcdf attributes

[mortenwh]

nansat.vrt.VRT.prepare_export_netcdf adds the string NANSAT_ to GCPProjection, Projection and GeoTransform metadata keys when the file is exported by nansat. Why is it done like this? It seems unnecessary and clutters the output.

GDAL does the same by adding GDAL_ to all global attribute keys. This should also be avoided. It seems good enough to have the GDAL attribute specifying version and release date, and we could do the same for nansat by adding a NANSAT attribute specifying version and release date.

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In the testfile nansat/tests/data/test_data/test.nc, the projection is defined by the key NC_GLOBAL#GDAL_NANSAT_GCPProjection. Why not just specifying it with a key GCPProjection?

[mortenwh]

@akorosov - would it be possible to follow up on this? It's so annoying when the attribute names are changed.... I also discovered that gdal creates a new history attribute, so we get two attributes with the history; (1) GDAL_history and (2) history. This is confusing.