BUG: measurement parser relative sigma definition for different component

[MohamedNumair]

I open this issue to fix it, I will remove the hardcoded sigma choice in

PowerModelsDistributionStateEstimation.jl/src/io/measurement_parser.jl

Line 153 in a8caad5

sigma = meas_var in ["vm","vr","vi"] ? σ/3 : σ/5/3 ;

and instead streamline a sigma value that is relative to the measurement value and its converted to its per-unit base.

[MohamedNumair]

as per commit 8f11df9 usage

σ_meas = Dict(
    "load" => Dict("load" => 2*0.01/3, "bus" => 0.01/3),
    "gen"  => Dict("gen"  => 2*0.01/3, "bus" => 0.01/3),
)