Bug in qepy/supercell

[palful]

The supercell class needs more checks in order to correctly read input and write output when non standard options are present

• In the supercell class, the new celldm(1) must be printed in the output if the atomic positions are given in alat units as is the default right now. Another possibility is to write them in angstrom or bohr.
• There is no support for ibrav=0 but now warning either. Support must either be introduced or at the very list an ibrav check must be added.