units 1.0-1

• Add internal workaround for udunits2 bug with parsing of units that contain
  "per", e.g. "percent" or "person"; #414

• Update minimum Rcpp version; #421

• Fix print methods for units and mixed_units, which now correctly return
  invisibly the object being printed; #422

• Wrap all pointers to avoid memory leaks derived from errors.

• Check for exception propagation in udunits2; #423

• Fix ud_convert() for empty input vectors; #428