Add the ability to automatically choose an initial zoom level and location for a map based on its layer(s)

[prushforth]

per a suggestion from Keith Pomakis

[pomakis]

I think the best way to accomplish this is to provide a clear way for a MapML layer to indicate its spatial extent. (At the moment there doesn't seem to be one.) Then, the client could simply use the union of the layers' spatial extents as its initial view.