implemented converted date for dates from other calendars

[rlskoeser]

preliminary support for calendar conversion:

• Need to convert to common calendar for comparison; probably use convertdate package; propose working with Hijri calendar as first case since it's used on two of our projects
• Need to know what it was converted from (since it could impact certainty)
• Conversion needs to be reversible

proposed implementation:

• create a new ConvertedUndate class that wraps the base Undate object and adds logic to convert between calendars and store the original calendar type and date (maybe also need interval variant?)

notes & questions

• undate objects should be calendar aware
• can calendar conversion be implemented as a formatter? could we adapt existing geniza calendar conversion code ? could a single formatter handle multiple calendars?

[robcast]

For my ISMI data I do need to keep the information which calendar the date was entered in (see #17)

I am not sure if we need to save the date in the original calendar as long as it can be converted back from the Gregorian datetime. In my sample the Hijri dates are converted losslessly to Gregorian and back but the converted date notes that the Gregorian date may be off by up to 2 days (which doesn't matter if you just use the Hijri).