feat(v4): PR 3c of the 4.0 cut - remove the ImputationDiD/TwoStageDiD/ContinuousDiD fit-time keywords and relay the bootstrapped surfaces

feat(v4): PR 3c of the 4.0 cut - remove the ImputationDiD/TwoStageDiD/ContinuousDiD fit-time keywords and relay the bootstrapped surfaces #628