(math/R-units) Updated 0.7.2 to 0.8.1

# version 0.8-1

* fix `%/%` and `%%` if arguments have different units; #313

* fix multiplier parsing for `exp(log(x))` operations; #321

* fix specification of secondary axes with `scale_units`; #326

# version 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

(mef)