Update methodology

[juaristi22]

This involves:

• more detailed explanation of microimpute and its benchmarking routine (in relationship to the experimental setup for SCF-CPS net worth imputation)
• overview of Supplemental Security Income and how it can be used to test the effect of different net worth imputations on downstream policy analysis, explanation of PolicyEngine's microsimulation used
• appendices 1 & 2 covering 1) benchmarking results on 6 other datasets, and 2) robustness check on the SCF-CPS imputation