Don't screw up absolute paths on Windows.


git-svn-id: https://svn.salilab.org/imp/trunk@17286 e91f8c7b-0e32-4433-a9ac-4502d39a4eab
add support for non-rigid members (issue #236)

These are members that have internal coordinates and so move with the rigid body, but aren't rigid (their internal coordinates are expected to change). As a result they don't contribute to the RigidBody derivatives and don't use the rigid body collision detection acceleration structures.

git-svn-id: https://svn.salilab.org/imp/trunk@17287 e91f8c7b-0e32-4433-a9ac-4502d39a4eab