Computation of Wolf, Reiher, and Hess's Douglas-Kroll-Hess relativistic
correction at 2nd-4th order wrapped in CMake for the Psi4 project.

It is based on these publications:
* A. Wolf, M. Reiher, B. A. Hess, J. Chem. Phys. 117, 9215 (2002);
  http://dx.doi.org/10.1063/1.1515314
* M. Reiher, A. Wolf, J. Chem. Phys. 121, 10945 (2004);
  http://dx.doi.org/10.1063/1.1818681