WCST_Import_Specification of unit of measurement (uom) for a coverage

[Bang Pham Huu]

Copied from ​http://earthserver.eu/trac2/ticket/229

for netCDF and GRIB which can contain metadata for units, user can specify the real value from file, with netCDF it will extract from variable's metadata, e.g:

{netcdf:variable:LAI:units} with LAI is an example of variable name

with GRIB, it can be something else (as it has multiple properties for units) but can be extracted from file's metadata, e.g:

${grib:unitsOfFirstFixedSurface}

the metadata is returned as a string, e.g: "ug/m3", "Celcius",… instead of 10^{1 as current.}

So, the new added property in general recipe is "units" and values from netCDF, GRIB file as above.

Then, when DesribeCoverage, the band (range) will have the specified units, e.g:

<swe:uom code="ug/m3"/>