Add support for approximate dates

[gaurav]

We need to be able to support dates that are approximate to a particular year, e.g.

https://github.com/cancerDHC/data-model-harmonization/blob/79849628bca909ef113a6031bf1eb25498c48ee2/data-examples/f2f-2021-09-data-examples/d1_harmonized_gdc_specimen_cc.yaml#L312-L313

However, we currently code this field as a linkml:datetime, which has to be in the same format as xsd:dateTime, so specifying just a year (or a year-month) is not allowed.