Paired eras

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).

The issue names no files, tests, or entry points. Start by reviewing how eras and year vectors are currently represented, then decide whether paired eras belong in analytical definitions or only in printing and plotting; done means one of those scopes is clearly selected and implemented with matching behavior.

Written by the indexing model from the issue text.