add angle-torsion potential, generalize stretch-torsion potential

git-svn-id: svn://dasher.wustl.edu/tinker@18 9dbd3ec4-0870-4ad8-bd6a-181c4f7deb04
reinstate Intel compiler-specific calls near the top of initial.f; remove/comment for other compilers

git-svn-id: svn://dasher.wustl.edu/tinker@19 9dbd3ec4-0870-4ad8-bd6a-181c4f7deb04
Add XYZEDIT option to create linear chain connectivity

git-svn-id: svn://dasher.wustl.edu/tinker@20 9dbd3ec4-0870-4ad8-bd6a-181c4f7deb04