New siesta
Fix of a serious bug, a missing unit conversion in the forces and stress. Two interface changes.
- Now the user can chose to write xc=('LDA', 'PZ') which is how the variable is stored internally.
- I found it inconvenient to write n_nodes=8 in the actual script, since the script is better if it is more machine-independent. Now the user specifies how siesta should be invoked rather than just the executable.