add TrueAttributeField

The TrueAttributeField only has the options "Yes" and "Unknown". This is
useful for events like having an illness:

Answering "No" is not useful because we cannot know if the event occured
in the meantime. One the other hand, the event can never be undone once
it happened, so "Yes" is a useful answer.

There are some edge-cases where it is not clear if his field is
applicable. Consider a pacemaker: Once you have it, you will typically
have it for the rest of your life. However, there is a theoretical
possibility that a future invention will make it obsolete.