units 0.8-0

• enhance unit mapping for newly installed units; #290

• remove deprecations: install_symbolic_unit, remove_symbolic_unit,
  install_conversion_constant, install_conversion_offset; #290

• fix multipliers for round trip log-exp operations; #292

• integrate ggplot2 scales (previously in the ggforce package) to
  automatically print axes with units; #294 addressing #164

• fix all.equal.units for non-units current

• fix zero power; #285

• fix unique.units to support arrays and matrices, implement methods for
  duplicated and anyDuplicated

• fix plot labels with spaces; #298 addressing #297

• always add units to labels, including user-provided ones; as part of #298

• new symbols/names with a percentage character are not allowed due to an
  upstream bug; #289