integrate USER-MOP package into build and documentat system. apply latest LAMMPS programming style conventions

- improvements documentation (dmi and exchange)
- correction error cross product in pair_spin_dmi.cpp
- implementation mech. part in pair_spin_dmi.cpp
- correction in all pairs: init_one for [j][i] couples
- correction in atom_vec_spin.cpp: index error in read_data
- some improvements in pair_spin_dmi.cpp and pair_spin_magelec.cpp

updated ScaFaCoS examples, added open-boundary Hammersley distributed sphere and charged cloud near a charged wall examples

- created README in examples/SPIN
- modified doc/src/set.txt to define 'spin' and 'spin/random' keywords