durations.css – correct leap-year duration; introduce olympiad (#475)

Hi there,

just noted an error in the durations. A leap year does not refer to a time span of four years, but to the length of the year where the leap day occurs, i.e. 366 days.

On the other hand, conveniently, there *is* a duration that describes a four-year timespan, an olympiad, which I introduced as a new property.

src/extra/durations.css

@@ -7,7 +7,8 @@
  --month: calc( 30 * var(--day));
  --quarter: calc( 13 * var(--week));
  --year: calc(365 * var(--day));
- --leap-year: calc( 4 * var(--year));
+ --leap-year: calc(366 * var(--day));
+ --olympiad: calc( 4 * var(--year));
  --decade: calc( 10 * var(--year));
  --generation: calc( 3 * var(--decade));
  --lifetime: calc( 8 * var(--decade));