WCS GetCapabilities user defined parameters

[pomadchin]

Add into WCS GetCapabilities custom parameters descriptions for the GetCoverage operation.

At this point our GetCatapbilities GetCoverage description may look like the following way:

<ows:Operation name="GetCoverage">
	<ows:DCP>
		<ows:HTTP>
			<ows:Get xlink:href="http://localhost:9000/"/>
		</ows:HTTP>
	</ows:DCP>
	<ows:Parameter name="service">
		<ows:AllowedValues>
			<ows:Value>WCS</ows:Value>
		</ows:AllowedValues>
	</ows:Parameter>
	<ows:Parameter name="AcceptVersions">
		<ows:AllowedValues>
			<ows:Value>1.1.1</ows:Value>
		</ows:AllowedValues>
	</ows:Parameter>
	<ows:Parameter name="InterpolationType">
		<ows:AllowedValues>
			<ows:Value>nearest neighbor</ows:Value>
			<ows:Value>bilinear</ows:Value>
			<ows:Value>bicubic</ows:Value>
		</ows:AllowedValues>
		<ows:DefaultValue>nearest neighbor</ows:DefaultValue>
	</ows:Parameter>
</ows:Operation>

We can add any arbitrary parameters like <ows:Parameter name="zFactor"> or <ows:Parameter name="azimuth"> into this description.

This task is to expose Vertical Exaggeration and Scale Factor as parameters in MAML Hillshade and Slope directives through GetCapabilities.