Support arithmetic functions

[slafayIGN]

Feature Request

Someone tried to use an SLD with the following rule in one of our API that relies on GeoServer. The goal is to make a style based on a ratio between two property values. You can imagine there were several similar rules in the SLD file.

<Rule>
	<Name>25 à 36%</Name>
	<ogc:Filter
		xmlns:ogc="http://www.opengis.net/ogc">
		<ogc:And>
			<ogc:PropertyIsGreaterThan>
				<ogc:PropertyName>"men_pauv" / "men" * 100</ogc:PropertyName>
				<ogc:Literal>25</ogc:Literal>
			</ogc:PropertyIsGreaterThan>
			<ogc:PropertyIsLessThanOrEqualTo>
				<ogc:PropertyName>"men_pauv" / "men" * 100</ogc:PropertyName>
				<ogc:Literal>36</ogc:Literal>
			</ogc:PropertyIsLessThanOrEqualTo>
		</ogc:And>
	</ogc:Filter>
	<PolygonSymbolizer>
		<Fill>
			<CssParameter name="fill">#ff5e23</CssParameter>
		</Fill>
	</PolygonSymbolizer>
</Rule>

But GeoServer doesn't accept the arithmetic operations in the PropertyName. So we had to replace it with specific SLD functions ogc:Mul and ogc:Div (reference is here: https://docs.geoserver.org/main/en/user/styling/sld/reference/filters.html#filter-expressions):

<Rule>
	<Name>25 à 36%</Name>
	<ogc:Filter
		xmlns:ogc="http://www.opengis.net/ogc">
		<ogc:And>
			<ogc:PropertyIsGreaterThan>
				<ogc:Mul>
					<ogc:Div>
						<ogc:PropertyName>men_pauv</ogc:PropertyName>
						<ogc:PropertyName>men</ogc:PropertyName>
					</ogc:Div>
					<ogc:Literal>100</ogc:Literal>
				</ogc:Mul>
				<ogc:Literal>25</ogc:Literal>
			</ogc:PropertyIsGreaterThan>
			<ogc:PropertyIsLessThanOrEqualTo>
				<ogc:Mul>
					<ogc:Div>
						<ogc:PropertyName>men_pauv</ogc:PropertyName>
						<ogc:PropertyName>men</ogc:PropertyName>
					</ogc:Div>
					<ogc:Literal>100</ogc:Literal>
				</ogc:Mul>
				<ogc:Literal>36</ogc:Literal>
			</ogc:PropertyIsLessThanOrEqualTo>
		</ogc:And>
	</ogc:Filter>
	<PolygonSymbolizer>
		<Fill>
			<CssParameter name="fill">#ff5e23</CssParameter>
		</Fill>
	</PolygonSymbolizer>
</Rule>

GeoStyler SLD parser doesn't recognize those functions. A quick try empties the filter.

That would be a useful addition.

I'm not sure this is a GeoServer vendor option. In that case it can be referenced as a child request of #969 .