Finished changes to attributes and attribute editor. The collection Attributes is now a (shallow (for now)) tree, instead of just a list. Child attributes are just used for uncertainty for now, and are used so the importer understands relationships between related attributes. Generally speaking, child attributes shouldn't show up in the UI. (I changed the default view for the CoreBrowser's grid to not display child attributes, for example.). Also some minor changes elsewhere to support importing asymmetric uncertainty.
A few more improvements to import_samples to take better advantage of the improvements to Attributes.