Allow to define the scale of an annotation label

[johann-petrak]

It should be possible to define the scale used for each annotation label. This should be one of:

For single choice (scalar) labels:

• nominal: values are either equal or unequal
• ordinal: values can be equal or bigger/smaller
• interval: values can be subtracted or added in addition to what is possible for ordinal
• ratio: values can be multiplied and divided in addition to what is possible for interval
• hierarchical: if a scalar label comes from a hierarchy, there are metrics that specifically consider the distance of values within the hierarchy
• [not a label]: some fields are actually not labels where anything like IAA should get calculated, e.g. comments, reasons, confidences

There will also be the following issues related to this:

• multiple choice labels where the user can select an arbitrary subset of all the possible values
  • this would be one example of a non-scalar label
• having more than one field express something about a single annotated concept, e.g. label value and label confidence

[davidwilby]

Relates to #27

[johann-petrak]

Feel free to merge into #27 if that makes more sense - though defining/configuring the scale of a scalar label could also be relevant in general and is not necessarily related to IAA only.