The popup_attribute takes an attribute name and uses the attribute as the popup value for each feature in the layer.

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>

Update to the GeoJson and GeoPandas examples to include the popup attribute

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>

Update the geojson popup attribute to being a popup function which will build a _popupContent attribute on features if required.

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>

Test on the attribute being correct and the geojson producing correctly, and test on the function creating the correct _popupContent attribue on the feature.

Also update test_geo_json_str so that it includes the empty options parameters.

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>

Allow passing the popup_function through the choropleth shortcut on the map object (assuming the dataset is GeoJson)

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>

Update the notebooks to include a popup function for GeoJson and GeoDataFrames

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>

The actual population of the `_popupContent` attribute based on popup_function is now in the style attribute. This means that the `popup_function` and `popup_attribute` can be populated after the Geojson creation.

The `popup_function` is actually a feature property now to ensure that the `popup_attribute` is set correctly when the function is set after the fact.

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>

Update the geojson examples to show the popup attribute and function can be set after the fact on geojson features.

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>

Fix the indent to fix the comment that fails the travis build

Signed-off-by: Henry Walshaw <henry.walshaw@gmail.com>