Regress: Design document for Marginal Effects

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Design document for marginal effects in regression.

Includes the following:
- Computation for marginal effects for logisitic and multilogistic regression
- Computation for standard errors for marginal effects of logistic regression
- Bug fixes for a bunch of errors that prevented proper compilation
of the design doc.
- Moved Clustered Sandwich Estimators within Regression since it is
currently used in the context of regression methods.