Guessing sld version from file directly and not setting sldVersion as option

[pprev94]

Today the parser takes an SLD Version as a parameter.
If the parameter is not set, the default value is 1.0.0.
It would be a lot more practical to have the parser guess the version from the SLD content itself.

When file content is parsed, the object sldObject contains already version :

sldObject[0][":@"]["@_version"]

if this version is not authorized, an error is thrown. If the version is recognized, it can be set using set sldVersion.

[jansule]

solved in #926