adapter-mesh: refactored bbox calculation
adapter-mesh: refactored bbox calculation

o moved calculation to own method
o added xflow request for "boundingbox" property: now checked first to avoid
  possible computation using other attributes
o added triggering of bbox computation when mesh is created

ariyapour/xml3d.js,xml3d/xml3d.js,xml3d/xml3d.js,ariyapour/xml3d.js,Fiware/webui.Xml3d.js,Fiware/webui.Xml3d.js,CCLDESTY/xml3d.js,CCLDESTY/xml3d.js