Add an imputation method

[smishr]

Post-stratification and calibration methods might take some time to be integrated into Survey.jl.

Imputation based method(s) can be investigated and integrated more easily into the package. There is choice of several methods cell mean, regression etc. I think cell mean method can be implemented as an easy fallback?

Ref

Section 8.6 Imputation: Lohr - Sampling Design and Analysis

[smishr]

Impute.jl may be used as the tool to impute values into DataFrames

[nadiaenh]

Yes I was thinking the same, will use this