fix WPS processVersion attribute retrieval during DescribeProcess request

[fmigneault]

When a GetCapabilities response is obtained, attribute wps:processVersion of all process offerings are nested under wps:Process elements. This makes parsing of the "root element" as namespace wps for each process.

<wps:Capabilities service="WPS"
                  version="1.0.0"
                  xml:lang="en-US"
                  xsi:schemaLocation="http://www.opengis.net/wps/1.0.0 ../wpsGetCapabilities_response.xsd"
                  updateSequence="1">
    [...]
	<wps:ProcessOfferings>
		<wps:Process wps:processVersion="0.1">
			<ows:Identifier>raven</ows:Identifier>
			<ows:Title>
		</wps:Process>
 		[...]
	</wps:ProcessOfferings>	
	[...]
</wps:Capabilities>

On the other hand, the DescribeProcess response provides attribute wps:processVersion under element ProcessDescription of the retrieved process(es) (without wps namespace), making the "root element" namespace an empty string.

<wps:ProcessDescriptions xsi:schemaLocation="http://www.opengis.net/wps/1.0.0 ../wpsDescribeProcess_response.xsd"
                         service="WPS"
                         version="1.0.0"
                         xml:lang="en-US">
	<ProcessDescription wps:processVersion="0.1"
	                    storeSupported="true"
	                    statusSupported="true">
		<ows:Identifier>raven</ows:Identifier>
 		[...]
	</ProcessDescription>
</wps:ProcessDescriptions>

The Process class was instantiated by always picking the active namespace from the root element, whether it was wps or "", but wps:processVersion always has the wps namespace.

This PR resolves retrieval of processVersion attribute following a DescribeProcess request.

Ex before:

wps = WebProcessingService("<url>")
print([p.processVersion for p in wps.processes if p.identifier == "raven"])
# ['0.1']   # works because of 'wps:Process' 

process = wps.describeprocess("raven")
print(process.processVersion)
# None   # fails because of 'ProcessDescription' instead of 'wps:ProcessDescription'

With PR:

print(process.processVersion)
# "0.1"

[cehbrecht]

test failures not related to this PR.

[fmigneault]

@cehbrecht Thanks