Rename `SlotValueAnnotation` to `SpanAnnotation`?

[IKostric]

• Annotation is already slot-value annotation as it has fields slot and value. Therefore, SlotValueAnnotation is not a descriptive name of what the class does.
• SlotValueAnnotation adds start and end fields referring to the start and end index in an utterance. SpanAnnotation would be more descriptive of the implementation.

[NoB0]

A proposition is to have three annotation classes based on the idea of slot value pairs:

• Annotation that have a slot and value without pre-defined type
• SlotValueAnnotation that have a slot and a string value taken from the utterance
• SpanAnnotation has an extension of SlotValueAnnotation that also precise the position of the value in the utterance

Note that SlotValueAnnotation and SpanAnnotation may be combined with start and end positions optional

[kbalog]

For Annotation, we could use 'key' instead of 'slot' as that class would likely be used for other types of annotations that are not of a slot nature (e.g., user satisfaction).

I like the suggestion of integrating SpanAnnotation into SlotValueAnnotation.