[ENH] octopus.py: allow to use ASE_OCTOPUS_COMMAND env var so that e.g. we can use MPI...
With the current setting, the executable can only be 'octopus' i.e. no way to customize the executable. This change make the Calculator more compatible with ASE standards, i.e. let the FileIOCalculator class handle the 'command', still setting its default to 'octopus' (serial).