Fix compile problem due to use of FastRotorSearch without Eigen
Move OBForceField::FastRotorSearch from confsearch.cpp to forcefield.cpp as it does not depend on Eigen (unlike the other contents of confsearch.cpp). This should fix the compile problem (see the CDashboard).
Merge pull request #53 from baoilleach/moveFastRotorSearch

Fix compile problem due to use of FastRotorSearch without Eigen