Make NPT Langevin able to run anisotropic simulation #300
Description
NPT MD simulations allow to have fixed pressure, while the cell shape changes. At the moment only isotropic volume change is possible: the cell is scaled by a scalar factor, the angles stay the same and the ratio of length too. In order to observe phase transition, it's important that the cell can change shape (example: titanium hcp -> bcc transition around 1150K).
NPT Langevin comes from Grønbech-Jensen, N., & Farago, O. (2014). Constant pressure and temperature discrete-time Langevin molecular dynamics. The Journal of chemical physics, 141(19).
It's possible to implement a full anisotropic method (angles and length). In this paper, only the length degrees of freedom are describe. I suggest to have a look on LAMMPS implementation in C++: https://github.com/lammps/lammps/blob/develop/src/fix_press_langevin.cpp, which should be the inspiration of the current method.