cal summary functions ignore era

`min.cal` and `max.cal` both assume that the object passed to them uses BP-like years, i.e. min returns `max(year)` and max returns `min(year)`. This is confusing if they're used on dates with a CE-based era or negative BCE/BP years.

These and other functions should take into account the `era` attribute of `cal` objects.

Start by locating the min.cal and max.cal implementations and inspect how cal objects store and use their era attribute. Check related summary functions for the same assumption; done means summaries handle CE, BCE, and BP or negative years consistently.

Written by the indexing model from the issue text.