Instrumental variables

Toolset panel_iv: 2 tools.

Tool	What it does	Notes
`panel_iv_regression`	Linear instrumental-variables regression of a dependent column on exogenous and endogenous regressors with excluded instruments.
`panel_iv_compare`	Fits several instrumental-variables estimators (and OLS, which treats the endogenous regressors as exogenous) on the same specification and lays them side by side: coefficients, standard errors, t statistics and p-values per estimator, with observations, R², adjusted R² and F per estimator.

Inputs, limits and outputs are described in Fincept Panel. Full schemas: fincept_describe_tool.