cubicweb #2144255 suboptimal setting for gmap zoom/center [validation pending]

Currently the zoomlevel is hardcoded to 8 if not provided, and the center is the average of the markers' coordinates. It should be possible to compute them automatically to get all markers on the map.

prioritynormal
typeenhancement
done in3.14.3
load0.100
load left0.000
closed by#151058945234 gmap: set center and zoomlevel automatically (closes #2144255)