Make Evita more flexible

[marcverhagen]

We want to be able to let the toolkit run on text that already has event annotations in it, with or without attributes. Evita now ignores these events. It should be able to import these events and run attribute code on it. Some subtasks:

• Check how Evita deals with events in the input, it will generally not add an event on offsets that already have an event, but the behavior there is not well-defined

• Add an Evita flag that prompts it to import tags that are already available in the input. The main problem here is with those tags that do not fit neatly in the TarsqiTree instance (there will have to be a chunk that the event fits in).

• Make Evita deal with multi-token events

• Isolate the functionality that adds attributes to events

This may also involve thinking about issue #3.