Implement <Function name="in"> element functionality

[fhb1990]

I came across SLDs that use this function to style a layer when an attribute attains a value that occurs in some list. Example found:

<ogc:PropertyIsEqualTo>
	<ogc:Function name="in">
		<ogc:PropertyName>visualisatiecode</ogc:PropertyName>
		<ogc:Literal><![CDATA[11300]]></ogc:Literal>
		<ogc:Literal><![CDATA[11301]]></ogc:Literal>
		<ogc:Literal><![CDATA[11302]]></ogc:Literal>
	</ogc:Function>
	<ogc:Literal>true</ogc:Literal>
</ogc:PropertyIsEqualTo>

This function is not yet supported in the parser.