Add `att.locatable` (`@where`) to `<death>` and `<birth>`

[frederik-elwert]

Currently, the element <event> has the attribute class att.locatable, which consists of a single attribute, @where. However, the two elements <death> and <birth> lack this class.

The prose text of the guidelines describes <birth> and <death> as two specialized elements for particular events. Thus, it would be expected that they can hold the same attributes as the <event> element itself. And particularly, recording the place of birth and death makes immediate sense.

We are currently designing the data structure for a prosopography file, and there recording the place of death is required. Thus, we would make immediate use of this attribute.