Add class for periods

It would be useful to have a class for representing periods of years (i.e. the span between a start and end age), then we could do e.g.:

```r
neolithic <- yr_period(11.7, 6, "ka") # or yr_span()? yr_interval()?

yr(10000, BP) %in% neolithic
# [1] TRUE

seq(neolithic, by = 0.1)
# [1] 11.7 11.6 11.5 [...] 6.1 6.0 ka

# in {c14}
as_cal(neolithic, distribution = "uniform")
```