get_operator unit tests

[jagot]

Unit tests are lacking most everywhere, but one very obvious that's missing is one checking that the equations generated from an energy expression indeed corresponds to the energy expression. In multi-configurational problems, off-diagonal terms appear in the energy expression that on variation yield source terms in the equations for the orbitals. It should be checked that the operators constructed act on the correct orbitals and are multiplied with the correct mixing coefficients, &c.