Add contrast to the observed outcome automatically in printed output

[nt-williams]

LMTP Estimator: SDR
   Trt. Policy: (policy)

Population intervention estimate
      Estimate: 507.7538
    Std. error: 4.8903
        95% CI: (498.169, 517.3386)

Additive effect
    Observed: ...
          Effect: ...
    Std. error: ...
        95% CI: (..., ...)