Added Kolmogorov-Crespi potential

doc/src/Eqs/pair_kolmogorov_crespi_z.tex

doc/src/pair_kolmogorov_crespi_z.txt

doc/src/pairs.txt

examples/USER/misc/kolmogorov-crespi/data.bilayer-graphene

examples/USER/misc/kolmogorov-crespi/data.graphene-adsorbant

examples/USER/misc/kolmogorov-crespi/in.atom-diffusion

examples/USER/misc/kolmogorov-crespi/in.bilayer-graphene

examples/USER/misc/kolmogorov-crespi/log.atom-diffusion

examples/USER/misc/kolmogorov-crespi/log.bilayer-graphene

potentials/CC.KC

src/USER-MISC/pair_kolmogorov_crespi_z.cpp

src/USER-MISC/pair_kolmogorov_crespi_z.h