removed redundant variable.

only one rot file is needed since only one subspace is optimized at a time.
minor.

eliminate dependence on iteration without active orbitals when setting the energy.
eliminate another unused variable.

cold was a copy of the coefficient which was intended for use with step rejection; not presently used so it has been removed.
eliminate another unused variable
simplifying optimize_subspace interface.
further interface simplification for optimize_subspace
added a function to compute unitary rotation matrix for a given subspace in zcasbfgs.

eliminates some of the optimize_electron forks and should improve readability.
connectedness should be judged in molecule