Low-Level Reco with explicit SM and explicit freq. or u

[nHackel]

This issue would require one to add an explicitSMLoading parameter which just returns a given SM. One could also add support for explicitly given u. Explicit freq. are already available.

I think the current naming is DirectFrequencyLoading..., so something in the same vein should be used: DirectSystemMatrix, ....

One would then provide a DirectReco plan