Linear multistep methods

Currently there is some code duplication in sharpclaw solver:
lines 256-285 (SSP104 method) do the same with lines 364-396 (function for starting method (SSPRK104) for the linear multistep case).

The only difference is that in the SSP104 case s1 and s2 point to registers whereas in the ssp104 function s1 and s2 are State objects (since registers need to store the solution at previous times).

Do you think we should merge these two parts in one part for both RK and multistep methods or keep it separately?