RobotState: Made transforms and attached bodies mutable to ease const-ness.

PLEASE DO NOT CASUALLY MERGE THIS.

This is posted mostly for discussion as an easy way to view a proposed change.

The const versions of getGlobalLinkTransforms() previously didn't do the same thing as the non-const versions... the const versions did not update the transforms, thus giving wrong answers (in release build) or failing an assertion in debug build.

This pull request shows a possible way around this.

What do people think?