Relax condition of atom ordering

Currently `Inelastica` expects the atoms to be ordered in the following blocks:
```
[buffer-left] (optional)
[electrode-left]
[device]
[electrode-right]
[buffer-right] (optional)
```
but such constraints are no longer present in `transiesta`. In a step towards full multiterminal compatibility we could first relax the above for the standard two-terminal setup.