handle dates with missing information, e.g. due to corrupted or missing text

[rlskoeser]

comment referencing these lines in the tests:

  # should we infer unknown month? or raise an exception?
  # assert str(Undate(2022, day="2X")) == "2022-XX-2X"  # currently returns 2022-2X
  # assert str(Undate(2022, day=7)) == "2022-XX-07"   @ currently returns 2022-07

I could see a situation with a corrupted text where we legitimately only know the year and day of the month, but we can't do much with the additional day info in this case. I don't think it should error; it's also unlikely we can infer the month in this situation. Is it possible to bump to the known granularity (year) for calculation purposes, and leave the day in a string rendering of the undate?

Originally posted by @ColeDCrawford in #36 (comment)

questions:

• does it make sense to raise an exception if you try to parse or export a date like this in a format that doesn't support this level of partial information?
• what is the granularity of a date like this, where we know the year and day but not month? does year make sense, or do we need gradations of granularity (year is certain, day is certain but month is unknown so day is only not fully known)