Option to add encoding and schema to wps:Output

[ruester]

Similar to issue #539 there should be a way of specifying the desired encoding and schema for the output of a result of wps:Execute/wps:Output.

This is an example of the corresponding wps:Output element which is currently not possible to create with OWSLib:

<wps100:Output mimeType="text/xml" asReference="false" encoding="UTF-8" schema="http://quakeml.org/xmlns/quakeml/1.2/QuakeML-1.2.xsd">
  <ows110:Identifier xmlns:ows110="http://www.opengis.net/ows/1.1">selectedRows</ows110:Identifier>
</wps100:Output>

Therefore the output parameter of the method execute should have two more optional entries within a tuple: encoding and schema, which are added to the wps:Output element.

The problem with the current implementation: there is no difference in getting an output with a MIME type of text/xml from the QuakeML schema (for example) and 'text/xml' from the GML schema.