`<docDate>` should be in att.datable and lose its bespoke `@when`

[joeytakeda]

In reviewing #2045, @ebeshero, @trishaoconnor, and I were surprised by (and thankful for) @martindholmes ' comment that <docDate> has its own @when and is not part of att.datable. This seems like an error that can be fairly easily resolved.

[JanelleJenstad]

<docDate> also needs a @calendar attribute. It needs to move to the new att.calendarSystem

[JanelleJenstad]

Example: All early modern books have a year (if they have a date at all). If you want to be able to process any data from encoded title pages, you would need a @notBefore and @notAfter date in order to capture the period between March 25 YYYY and March 24 YYYY+1. You also need to be able to capture the @calendar used in the text node (e.g., Julian).

[raffazizzi]

Another example with 18--: https://digitalcollections.nypl.org/items/510d47e2-c586-a3d9-e040-e00a18064a99

[JanelleJenstad]

Another example:https://digitalcollections.nypl.org/items/510d47e2-c586-a3d9-e040-e00a18064a99 The date one would transcribe and then tag with <date> and @notBefore and @notAfter is 18--