More robust way of BBOX parsing

Some servers return:

<ows:BoundingBox dimensions="2" crs="http://www.epsg.org#4326">
<ows:LowerCorner> </ows:LowerCorner>
<ows:UpperCorner> </ows:UpperCorner>
</ows:BoundingBox>
Merge pull request #49 from jachym/patch-1

More robust way of BBOX parsing