Avoid warning with g++ -Wcast-qual by using const_cast to make world group object pointers writable.
Avoid warning with g++ -Wcast-qual by using const_cast to make world group object pointers writable.

git-svn-id: 67bf12f30ddf2081c31403e0b02b2123541f1680@22744 08b3d480-bf2c-0410-a26f-811ee3361c24

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