support partially known dates (#36)

* Support initializing undate with partially known year or month

Co-authored-by: Cole Crawford <[email protected]>

* Handle some cases for initializing undate partially known days

Co-authored-by: Cole Crawford <[email protected]>

* Add notes about next steps

Co-authored-by: Cole Crawford <[email protected]>

* Clean up known year logic; disable failing tests

* Refactor logic for calculating missing month digits

* Refactor logic for calculating missing day digits

* Refactor shared partially known month/day logic for min/max values

* Update duration logic for partially known dates with known granularity

* Update string methods for partially known dates

* Adjust partial date duration calculation for dates that wrap years

* Make duration logic more consistent

* Add jupyter notebook comparing partial date duration logic with S&co

* Pin sphinx to pre 7.0 for compatibility with rtd theme

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Co-authored-by: Cole Crawford <[email protected]>