Specifying cif2cell as a prerequisite

[jingsk]

lmf() as is written cannot be run unless cif2cell is installed. I installed it using pip install cif2cell as suggested here (https://github.com/torbjornbjorkman/cif2cell) and now lmf is working. I think stating cif2cell as a requirement and adding a line to install it in setup.py is probably a good idea.