brucefan1983/GPUMD: GPUMD-v2.5.1

Major changes compared to GPUMD-v2.5:

• Created Jupyter notebooks (with Python 3) as the new tutorials. The thermo package is used.
• Fixed a bug related to the ensemble npt_ber command. The bug will affect simulations where the z direction is periodic and not deformed and at the same time the target pressures for the z and x directions are different. The reason is that the unfix code will control the box size in the z direction according to the target pressure in the x direction.