quantities 0.2.0

• Add compatibility between errors::geom_errors (r-quantities/errors#52) and
  units::scale_[x|y]_units (r-quantities/units#294) (#13).
• Fix warning in as.list.quantities.
• Add plotting examples to introductory vignette.
• Add support for errors with units (#14). For objects of class quantities or
  units, the errors() method now returns a units object that matches the
  units of x. Methods errors<-() and set_errors() assume that the provided
  uncertainty (value) has the same units as x (for backwards compatibility).
  However, it is a best practice to provide a value with explicit units. In
  this way, uncertainty can be provided in different (but compatible) units,
  and it will be automatically converted to the units of x (see
  help("errors", "quantities")).
• Add support for correlations and covariances with units for objects of class
  quantities (as part of #14).
• Remove some superfluous .quantities methods (as part of #14).
• Implement methods for duplicated, anyDuplicated and unique (#12).