function propagator(U, dpar::DiracParam{T}, dws::DiracWorkspace,lp::SpaceParm, maxiter::Int64, tol::Float64, y::NTuple{4,Int64}, c::Int64, s::Int64)
Returns the fermionic progapator for a source at point `y` with color `c` and spin `s`. If the last three arguments are replaced by `time::Int64`, the source is replaced
by a random source in spin and color at t = `time`.
"""
function propagator(U,dpar::DiracParam{T},dws::DiracWorkspace,lp::SpaceParm,maxiter::Int64,tol::Float64,y::NTuple{4,Int64},c::Int64,s::Int64)where{T}