Documenting limitations of forEach and forEachInExtent

Because the tree's structure changes when nodes are inserted,
updated or deleted, no modifications are allowed in a forEach
loop.
Merge pull request #1321 from ahocevar/safe-foreach

Documenting limitations of forEach and forEachInExtent
Use ol.structs.RBush in ol.interaction.Modify

This is one more step to bring the master and vector-api
branches closer together. I am well aware that the current
Modify interaction will probably be rewritten as part of the
vector-api effort, but with changes like this we will eventually
be able to remove ol.structs.RTree from master.
Merge pull request #1322 from ahocevar/modify-rbush

Use ol.structs.RBush in ol.interaction.Modify
Throw an exception if an ol.structs.RBush is modified while reading
Only activate ol.structs.RBush conflict checks when goog.DEBUG is true
Rename ol.structs.RBush#all to getAll
Rename ol.structs.RBush#allInExtent to getAllInExtent
Fix infinite loop bug in ol.structs.RBush
Update ol.interaction.Modify to use ol.structs.RTree#getAllInExtent
Merge pull request #1324 from twpayne/rbush-improvements

RBush improvements
Merge branch 'master' into vector-api
Use rBush.getAll and rBush.getAllInExtent