Support LightDFG for solving

[GearsAD]

REF: JuliaRobotics/DistributedFactorGraphs.jl#67

[Affie]

@GearsAD it looks like it runs a lot deeper, I'll need to complete a course in IIF to fix.

got this so far to init with the same type :-)

#init an empty fg with a provided type
function initfg(fgtype::DataType;
                 sessionname="NA",
                 robotname="",
                 username="",
                 cloudgraph=nothing)::AbstractDFG

  fgtype <: AbstractDFG || @error "wrong type argument for initfg()"

  return fgtype(params=SolverParams())
end

in the mean time, this seems to fix it for now JuliaRobotics/DistributedFactorGraphs.jl#73
with IIF fix #367

[dehann]

Take this as lecture 1, intro into IIF

[dehann]

related to #364