Template Fock operator

[ryanmrichard]

The Fock operator is an effective one-particle operator and, like other operators, should indicate the particle in its type. Notably right now there is no type-safe way to distinguish between the Fock operators appearing on the left and right of this equation ($N_e$ being the number of electrons):

$$\hat{F}=\sum _{i=1}^{N_e}{\hat{f}}_i.$$