zoom and boundary limits

[sbsimo]

I think we should limit the bounding box of the map to the African continent plus some buffer, and set limits to the minimum and maximum zoom levels as well.

[ireneITHACA]

extent and MinZoom adjusted. The maximum zoom hasn't been touched for I thought it was good like it was...check it on my IP Simon before to commit the changes.

[ireneITHACA]

Actually the pan still needs to be limited! I'll do it on Monday :)

[ireneITHACA]

Now panning outside a certain extent is forbidden. However the extent boundaries are fixed and not adapted to different zoom levels. This implies the boundaries being big enough to let the user see the content on the edges of Africa at the maximum zoom level. In fact only the view center is constrained. I still have to work on this issue.

[ireneITHACA]

After careful investigation it seems that the issue encountered is well known but not yet fixed (openlayers/openlayers#3913 and http://stackoverflow.com/questions/27200086/restrict-pan-outside-wms-extent-in-openlayers3 and http://stackoverflow.com/questions/30726751/extent-bounds-not-respected-on-zoom). I suggest to wait for an official solution implemented in a newer OL3 release than to build a workaround.