Factorize code that expand a template url into multiples urls

This factorize code that turns url such as: ``'http://tile-{c-e}/{z}/{x}/{y}'`` into: ``['http://tile-c/{z}/{x}/{y}', 'http://tile-d/{z}/{x}/{y}', 'http://tile-e/{z}/{x}/{y}']``.

This PR also applies this ``expandUrl`` function to the TiledWMS source.