Green's function details need to be in the manual

Details relating to the QMCPACK Green's function need to be written up in the manual. This relates both to use (e.g. how to select different branch cutoff schemes, such as ZSGMA) and the technical implementation of the Green's function.

On the technical side, we should document at least at the math/formula level what our short time Green's function is and how user inputs modify it. We should also note where different terms in the formulation reside in the codebase.

Doing this will create more confidence in the code from QMC experts considering using QMCPACK and also make it easier to implemented future innovations relating to the Green's function.