adaptr 1.4.0

This is a minor release implementing new functionality, and including bug fixes,
updates to documentation, argument checking and test coverage.

Added the rescale_probs argument to the setup_trial() family of
functions, allowing automatic rescaling of fixed allocation probabilities
and or minimum/maximum allocation probability limits when arms are dropped
in simulations of trial designs with >2 arms.
The extract_results() function now also returns errors for each simulation
(in addition to squared errors) and the check_performance(),
plot_convergence(), and summary() functions (including their print()
methods) now calculate and present median absolute errors (in addition to
root mean squared errors).
The plot_metrics_ecdf() function now supports plotting errors (raw,
squared, and absolute), and now takes the necessary additional arguments
passed to extract_results() used for arm selection in simulated trials not
stopped for superiority.
Added the update_saved_calibration() function to update calibrated trial
objects (including embedded trial specifications and results) saved by the
calibrate_trial() function using previous versions of the package.
Rewritten README and 'Overview' vignette to better reflect the typical usage
and workflow.

The setup_trial() family of functions now stops with an error if less than
two arms are provided.
The setup_trial() family of functions now stops with an error if
control_prob_fixed is "match" and fixed_probs is provided for the
common control arm.
Improved error message when true_ys-argument is missing in
setup_trial_binom() or when true_ys- or sds-argument is missing in
setup_trial_norm().
Changed the number of rows used in plot_convergence() and plot_status()
if the total number of plots is <= 3 and nrow and ncol are NULL.
Fixed a bug in extract_results() (and thus all functions relying on it),
causing arm selection in inconclusive trial simulations to error when
stopped for practical equivalence and more simulated patients were
randomised than included in the last analysis.
Improved test coverage.
Minor edits and clarification to package documentation.
Added references to two open access articles (with code) of simulation
studies using adaptr to assess the performance of adaptive clinical trials
according to different follow-up/data collection lags
(https://doi.org/10.1002/pst.2342) and different sceptical priors
(https://doi.org/10.1002/pst.2387)

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