[SLD] external graphic and gap parameter

[assefa]

When external graphics are used on a line featire (symbol along a line), since sld 1.0 does not provide a gap parameter, the gap is fixed now to be size_of_the_symbol*2. This seems to give a reasonable ouptput but lacks flexibility for the user. SLD 1.1.0 supportd the gap parameter :  http://schemas.opengis.net/se/1.1.0/Symbolizer.xsd <xsd:complexType name="GraphicStrokeType?">)

It was suggested that until we upgrade to 1.1.0, we should leave the gap to 0 and force the user to create symbols that have "gaps" in them.