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 / able to round-trip

proposed implementation:

• initial approach will be to use Undate object without adding a new converted date type
  • store original date using the undate text label field
  • add explicit calendar logic; store original calendar, use consistent internal calendar for comparison
• implement calendar conversion as a "formatter" (probably rename these as converters to be more general)
• adapt existing Princeton Geniza Project calendar conversion code, but look into writing as a parser (similar to EDTF approach) to make it more extensible
• use ISMI sample data as a test case / comparison; write an example notebook with updated ISMI data (see: complex date objects from the ISMI database #17 ) and compare/demonstrate calendar conversion between the two implementations

[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).

[rlskoeser]

questions resolved based on discussion with @robcast ; notes: #95