- 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

- document previously undocumented OpenCL tune settings
- implement OpenCL platform selection through prefixing the device type with the platform id separated by a colon
- allow passing custom tune parameters though postfixing the device type with the 13 tuneable parameters separated by commas
- remove an extra clear() that would delete device properties structs an cause LAMMPS to output garbage strings

+ added examples for special cases with disks and rods for 2d, and spheres and rods for 3d,
+ corrected the definition of the cutoff distance in pair style command

Fixes the `pdf` target of the `Makefile`.