* The unit tests should reflect basic definitions.
* Derivative relationships do not need to be tested.
* Derivative tests needlessly increase test times and noise.
* After much consideration I consider this not a good change.
* It introduced a completely arbitrary mathematical relationship
among the units.
* This commit restores a precise relationship among the units which
is based on the Gregorian calendar.
* However this experiment has been preserved in the git history.
* The Gregorian average of months and years is an
integral number of seconds.
* This patch slightly tweaks the average to make it
not an integral number of seconds, and thus these
units no longer implicitly convert to seconds.
* This will flag naive mixing of years, months and seconds
as a compile time error.
