Update DBH estimation in ITSMe

[BarbaraDh]

• Added more robust DBH estimation through the how
  argument. The original ITSMe behaviour can be obtained with how = "mean";
  how = "median" (new default) uses the median point-to-centre radius; and numeric values
  use a trimmed mean of the point-to-centre radii.
• Added DBH quality-control metrics to diameter_slice_pc() and dbh_pc():
  arc_coverage and inner_circle_empty.
• Updated summary_basic_pointcloud_metrics_pertree() and
  summary_basic_pointcloud_metrics() so DBH quality-control metrics are
  included in the returned summary data frame when "stem diameter" is
  calculated.
• Updated the lower-trunk fallback logic in dbh_pc() so that a well-supported
  direct DBH slice is less likely to be overwritten because of noisy or
  incomplete lower-stem points.
• Documented the new DBH estimation options and quality-control metrics in the
  main ITSMe vignette.