Commit JT 042018, new spin/precession and pair/spin (peru virtual)

doc/src/Eqs/pair_spin_dmi_interaction.jpg

doc/src/Eqs/pair_spin_dmi_interaction.tex

doc/src/fix_precession_spin.txt

doc/src/pair_spin_dmi.txt

doc/src/pair_spin_exchange.txt

doc/src/pair_spin_me.txt

doc/src/pair_spin_neel.txt

examples/SPIN/bfo/in.spin.bfo

examples/SPIN/cobalt_fcc/in.spin.cobalt_fcc

examples/SPIN/cobalt_hcp/in.spin.cobalt_hcp

examples/SPIN/iron/in.spin.iron

examples/SPIN/nickel/in.spin.nickel

src/SPIN/fix_nve_spin.cpp

src/SPIN/fix_nve_spin.h

src/SPIN/fix_precession_spin.cpp

src/SPIN/fix_precession_spin.h

src/SPIN/pair_spin.cpp

src/SPIN/pair_spin.h

src/SPIN/pair_spin_dmi.cpp

src/SPIN/pair_spin_dmi.h