cal summary functions ignore era

[joeroe]

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.

[joeroe]

I think what should happen here is:

• min.cal() and max.cal() should behave like the base versions, i.e. return the numerical min and max
• era should provide yr_earliest() and yr_latest() that responds to the direction of the era (Add yr_earliest() and yr_latest() era#43)
• c14 should provide cal_age_range() that uses yr_earliest() and yr_latest()