ENH: Add UnitCellFilter stress/strain Jacobian

UnitCellFilter has been modified such that the generalized stress/strain DOF are modulated by a Jacobian in order to ensures that these DOF scale in the same way as the atomic positions and forces. Specifically, the the strain Jacobian is V^(1/3) N^(1/6), where V is the unit cell volume and N is the number of atoms in the unit cell. The stress Jacobian is -V^(2/3) N^(-1/6). In addition, a "weight" argument has been added to UnitCellFilter which allows the user to modulate the importance of the unit cell DOF relative to the atomic DOFs. This affects both the step size and the force-based convergence criterion used in ASE optimizers.

See http://dx.doi.org/10.1063/1.3684549 for more details.