Interactive feedback for map annotation

[jc-healy]

Given a map with an annotation layer it would be a very nice feature be able to provide feedback into regions of the map you'd like more detail in annotating.

A few ways to do this off the top of my head (increasing in level of difficulty):

1. provide a simple "add annotation" button to the various summarizer widgets. Perhaps better would be a new add_annotation_button widget that uses many of the same functions under the hood as the JointWordCloudSummarizer or JointLabelSummarizer. This doesn't work super well for layered labels but would be great for quickly annotating a plot for static presentations.

2. Slightly better for less local annotators such as the SparseMetadataLabelLayers which depend on a clustering of the data might be to allow a user annotated clustering to be passed in. Not ideal because it won't handle the hierarchy well but it at least allows a user who understands the clustering and labelling to make use of the annotator without having to delve into the depths of how it works.

3. More longer term but interesting would be to incorporate the semi-supervised clustering into the various annotators such as JointVectorLabelLayers and SparseMetadataLabelLayers. This would allow the full construction of a hierarchical cluster model to create layers that were more consistent with the interests of a user.