modelbased 0.16.0

Changes

• Informative error message when the trend variable in estimate_slopes() is
  not numeric and backend = "emmeans".

• estimate_contrasts() gets a post_process argument, to process subsequent
  comparisons. It allows for complex, multi-step comparisons.

• The comparison argument in estimate_contrasts() can now also be "omnibus",
  to calculate a global omnibus test for differences between levels of a
  predictor.

• Offsets (using the offset argument) now also work when estimate is set to
  "population" or "average".

• Argument slope, when specific values are defined, can now also be a list
  instead of only a string. E.g., slope="<variable> = seq(1, 3, 0.1)" can now
  also be written as slope=list(<variable> = seq(1, 3, 0.1)).

• Clarified usage of marginal group-level estimates in estimate_grouplevel().
  The type argument can now be used in combination with the estimate argument,
  which is now also more explained in detail in the documentation.

• by = NULL now also works for backend = "emmeans", to predict the grand mean.

Bug fixes

• Minor bug fix when evaluating slope and by arguments in estimate_slopes().