Paired eras

[joeroe]

CE-based eras come in pairs, i.e. negative values of CE are BCE; negative values of BCE are CE. There is currently no concept of this in era. I can think of two ways to add it:

1. As part of the definition of an era, e.g. add an inverse attribute that contains another era.
2. As a function, which takes a year vector with negative values and splits it into two positive vectors with paired eras.

The underlying question is whether it makes sense to support paired eras in analytical use (in which case we'd need 1) or just for printing and plotting (then 2 would be enough).

[joeroe]

Along with this, could add something like yr_earlier() and yr_later() to add and subtract years respecting their direction and paired era.