implement labels for undate and undate interval

[rlskoeser]

there's a placeholder label attribute for undate, but we aren't doing anything with it yet

proposed initial implementation:

• add label field to UndateInterval
• for both Undate and UndateInterval, add optional label parameter to init method that sets the label
• for both, if label is set then it should be included in the string representation and repr; maybe something like label (dates) if label is defined

[jdamerow]

completed in https://github.com/dh-tech/undate-python/pull/52