Enable openmp support for opencamlib

Can you enable openmp support for opencamlib? I tested and it works fine. Change the cmake line from:

cmake -D BUILD_PY_LIB=ON -D BUILD_CXX_LIB=OFF -D USE_PY_3=ON ../src

to:

cmake -D BUILD_PY_LIB=ON -D BUILD_CXX_LIB=OFF -D USE_PY_3=ON USE_OPENMP=ON ../src