Add compatibility utility for nholthaus units

[chiphogg]

This creates a file, "compatibility/nholthaus_units.hh", which makes
it easy to set up a correspondence between each Au Quantity type, and
the corresponding nholthaus units type. This "correspondence" includes
bidirectional implicit conversions: so, you can pass
::au::QuantityD<Meters> to an API expecting
::units::length::meter_t, and vice versa.

This correspondence is purely opt-in. The file also needs to be
included after both of the other two libraries are included. Since
this is fragile in general, we give instructions and an example for how
to make it not fragile, in nholthaus_units_example_usage.hh.

We include a variety of unit tests to show that the quantities are
indeed equivalent between the libraries. (Actually, this isn't quite
always the case. These tests uncovered a bug in the nholthaus library,
nholthaus/units#289.)

We reserve the documentation of the corresponding-quantity feature in
general, as well as its application to the nholthaus library, for a
future PR.

Test plan

• Add new unit tests for this file.
• Add example file that shows how to use it.
• Test that the file can replace Aurora's existing implementation.

[geoffviola]

I only did a quick review. The usage header gives some notes on how to #include it and the tests provide sample conversions. Seems useful for migration. Thanks!