compilation of branch OPAL-1.6 fails if DKS is enabled

If DKS is enables compilation fails on the OPAL-1.6 branch:

/opt/tmp/gsell/src/OPAL-1.5.1-20170210/src/Classic/Solvers/CollimatorPhysics.cpp: In member function ‘void CollimatorPhysics::setupCollimatorDKS(PartBunch&, Degrader*, size_t)’:
/opt/tmp/gsell/src/OPAL-1.5.1-20170210/src/Classic/Solvers/CollimatorPhysics.cpp:1081:47: error: ‘class Degrader’ has no member named ‘getZSize’
         double params[numpar] = {zBegin, deg->getZSize(), rho_m, Z_m,

Without DKS it compiles without problems.