Add print S3 methods for any classes missing them (e.g., EpiRange)

[brookslogan]

The highest-priority things to get print methods already have methods written, but may not be used; see #43. However, classes like EpiRange may still benefit from a neater-looking print method.

[capnrefsmmat]

Seconding that EpiRange in particular needs a print method. For daily and weekly data, they currently print like this:

epirange(as.Date("2021-01-01"), as.Date("2021-01-07"))
#> $from
#> [1] 20210101
#> 
#> $to
#> [1] 20210107
#> 
#> attr(,"class")
#> [1] "EpiRange"

epirange(202202, 202204)
#> $from
#> [1] 202202
#> 
#> $to
#> [1] 202204
#> 
#> attr(,"class")
#> [1] "EpiRange"

I think there should be a simple human-readable format, that (a) clearly states if it's daily or weekly, (b) puts the dates in a standard format like ISO 8601 instead of mashing components together with no spacing, and (c) is nice and compact on one line.