Pack/unpack should respect the estimate type.

[GearsAD]

REF: https://github.com/JuliaRobotics/DistributedFactorGraphs.jl/blob/master/src/services/DFGVariable.jl#L20-L21

[Affie]

I think this revers to:

DistributedFactorGraphs.jl/src/services/Serialization.jl

Line 28 in 63a96ca

ppeDict = JSON2.read(packedProps["ppeDict"], Dict{Symbol, MeanMaxPPE})

We currently only have the one PPE type.