add frozen HS_WCA
Merge branch 'master' of https://github.com/smcantab/pele
Merge branch 'master' of https://github.com/smcantab/pele
add some cpp tests for HS_WCAFrozen
add cpp tests for HS_WCAPeriodicFrozen
undo typo change to file
add test of frozen property
Merge remote-tracking branch 'upstream/master' into frozen_hs

Conflicts:
	cpp_tests/source/test_main.cpp
	source/pele/frozen_atoms.h
add hessian tests for HS_WCAFrozen, add get_energy_gradient_hessian for frozen_atoms
Merge pull request #22 from kjs73/frozen_hs

add potential for WCA hard sphere with frozen atoms