Change summary behaviour for `compute_extrapolation` and related functions

[dill]

This makes the following changes to dsmextra:

• compute_extrapolation()/summarise_extrapolation():
  • remove argument print.summary (superceeded by new functions)
  • remove argument print.precision (superceeded by digits argument to print method)
  • remove argument save.summary (superceeded by new functions)
  • function now always returns the $summary element
  • new functions summary.extrapolation_results() and print.extrapolation_results_summary() to access and print the `$summary object
• extrapolation_analysis()
  • remove argument summarise.extrapolation
  • remove argument summary.print.precision
  • (from changes above)
• summarise_extrapolation
  • remove argument print.precision (superceeded by digits argument to print method)
• documentation updates
• changes to vignette and tests for the above changes