Implement off-diagonal projectors

[TysonRayJones]

QuEST so far supports diagonal projectors $\ket{i}\bra{i}$ as is useful in modelling measurement. E.g.

applyMultiQubitProjector(qureg, targs, outcomes, ntargs);

However, off-diagonal projectors like $\ket{i}\bra{j}$ (where $i \ne j$) might also be useful to effect directly upon density matrices.

multiplyOffDiagonalMultiQubitProjector(qureg, targs, ketOutcomes, braOutcomes, ntargs);

Certainly many decoherence processes can be described by off-diagonal projectors, and would make natural jump operators in the considerations of #651.

Effecting |i><j| sees amplitudes consistent with the j substate become zeroed, while amplitudes instead consistent with the i substate become overwritten by the aforementioned amplitudes; all remaining amplitudes are zeroed. We must sketch and check whether the movement of amplitudes is compatible with our distribution scheme (but I intuit it is).

Since off-diagonal projectors are nilpotent, they also have trivial gadgets $exp(x \ket{i}\bra{j}) = I + x \ket{i}\bra{j}$ to match those proposed in #655.