Add support for multi-valued input in WPS4R

[nuest]

Feature

The WPS standard allows input values to occure multiple times with the same name (see also here), e.g.

[...]
        <wps:Input>
          <ows:Identifier>levels</ows:Identifier>
          <wps:Data>
                <wps:LiteralData>1200</wps:LiteralData>
          </wps:Data>
        </wps:Input>
        <wps:Input>
          <ows:Identifier>levels</ows:Identifier>
          <wps:Data>
                <wps:LiteralData>1400</wps:LiteralData>
          </wps:Data>
        </wps:Input>
        <wps:Input>
          <ows:Identifier>levels</ows:Identifier>
          <wps:Data>
                <wps:LiteralData>1600</wps:LiteralData>
          </wps:Data>
        </wps:Input>
[...]

Definition of done

• evaluate how WPS API handles this internally (might need to add support for this?)
• create test script and IT
• extend wps.in annotation > it must create a vector of inputs if multivalued
• research how this works for non-literal data in the WPS
• create demo script and execute-request