set minimum PME grid dimension (16) for use with very small unitcells

git-svn-id: svn://dasher.wustl.edu/tinker@56 9dbd3ec4-0870-4ad8-bd6a-181c4f7deb04
safeguard LBFGS optimizer for use with steepest descent method

git-svn-id: svn://dasher.wustl.edu/tinker@57 9dbd3ec4-0870-4ad8-bd6a-181c4f7deb04