Bug when PBC and Atoms are outside the cell

[cortner]

at the moment fix_cell provides a workaround, but doesn't fix the bug. See JuLIP.jl:#99 an example to reproduce this.

[jameskermode]

Just ran into this again with some non-periodic cells where cell was set to the zero matrix. _fix_cell_() doesn't seem to cope with this case. For now I fixed by hand but will see if I can come up with a general solution.

[cortner]

I wonder whether there is scope to just ignore the cell argument entirely and compute it an the fly if PBC is not used.