duration for uncertain years are calculated based on earliest/latest days

[rlskoeser]

Undate duration logic for uncertain years hasn't been implemented properly.

At a minimum this should return something like the default or most common number of years for the given calendar, rather than calculating the number of days from the earliest possible date to the last possible date in range.

Once we have support for uncertain integers (#129), then this should be updated to return a undelta based on the min/max days for possible years in range for the current calendar.

>>> from undate import Undate
>>> Undate("19XX").duration().days
36524
>>> Undate("190X").duration().days
3652
>>> Undate("190X", calendar="Hebrew").duration().days
3661