This method generates the descriptive form.
In addition to the internal form used by the system Units,
UnitConstraints, UnitExprs, etc. have a descriptive form that is used by
humans. This method generates that form so that humans can
easily understand the results generated by the system. For example, an
internal representation of a Unit could be 4.1868E7*<2, 1, 1, 0, 0>
and its descriptive form would be "calorie".
The descriptive form is invariant across Unit Systems. That is, calorie
is calorie regardless of the Unit System. But, the internal form may be
4.1868E7*<2, 1, 1, 0, 0> in one Unit System, and
1.0*<2, 1, 1, 0, 0> in another. Therefore the descriptive form is
used as the external representation. This makes it
possible to save a model under one Unit System, and then load it under a
different Unit System.