reviser 0.2.0

Bug fixes

• summary() on a long-format tbl_pubdate no longer fails with
  "character string is not in a standard unambiguous format". The method
  assumed a wide layout and treated the pub_date and value column
  names as publication dates, so it failed on every get_revisions()
  result. The reported number of time periods and vintages was also wrong
  for long input.
• print() and summary() on a kk_model now report which specification
  was estimated. model = "Howrey" and model = "Classical" previously
  produced identical headers, because the fitted object never recorded the
  model argument. jvn_model objects likewise report whether news,
  noise or both were estimated.

New features

• kk_model and jvn_model objects gain the standard extractor methods:
  coef(), vcov(), logLik(), nobs(), fitted(), residuals() and
  predict(). AIC() and BIC() therefore work, and reproduce the values
  shown by summary().
• New states() generic to access the estimated state paths of a fitted
  revision model, replacing direct use of fit$states.
• New validate_vintages() checks a tbl_pubdate or tbl_release object
  against the documented class contract. See ?"reviser-vintages-classes"
  for the contract itself.

Internal

• Covariance matrices are obtained through a Cholesky factorization, which
  exploits the symmetry of the Hessian and reports when it is not positive
  definite instead of silently applying a ridge. Delta-method
  transformations exploit the diagonal structure of the Jacobian, and the
  Kalman recursions use tcrossprod(). Estimates are unchanged.