tools for working with itksnap label descriptions

[NileGraddis]

ITKSnap is commonly used to view CCF annotation labels and registered data. In order to view CCF annotation labels in ITKSnap properly, two things need to happen:

1. A label description file needs to be generated based on a structure set (using this term literally, not with reference to the warehouse model).
2. If the structure set includes elements with id > 2**16-1, the ids need to be remapped to lower values (ITKSnap has trouble with large label values). We need to be able to generate an annotation with ids remapped to these lower values, as well as maps between the id sets.