Node: clarify naming: node's position -> node's origin

getWorldPosition -> getWorldOrigin
setWorldPosition -> setWorldOrigin
getRelPosition -> getRelOrigin
setRelPosition -> setRelOrigin
Note: old names remain as alias for backward compatibility.
Node: make getSRT() more robust if the node only has a matrix.
Merge branch 'master' of https://github.com/PADrend/MinSG