upgrade Mixture getSample to allow n-ary Tuple of values

[dehann]

Currently on first element of meas::Tuple is passed through getSample(::Mixture{...}, N ). https://github.com/JuliaRobotics/IncrementalInference.jl/blob/444b4f02c9861ed5a37c93cf50213532b64c2d50/src/Factors/Mixture.jl#L86-L87

Defer until after #927, #467 are complete.

Tangentially related xref #1096

[Affie]

We should reevaluate this issue to fit with #1371.