Add note-well box to clip data before calculating Gi* values where necessary

[mpjashby]

Since Gi* is a relative measure, if you have data for a large area (e.g. a country) but only want to show data for a smaller area (e.g. a city), the Gi* values will be influenced by the large areas with no crime and all of the city is likely to be identified as a hotspot. To prevent this, it is important to clip the dataset before calculating the Gi* values, as well as then clipping afterwards where necessary.