Amend disease extent generation to support disease group set-up, as follows:

The disease extent generation process needs to be changed to support disease group set-up, as follows:

1. Initial disease extent generation now uses all disease occurrences from all years.
2. Updating a disease extent now uses data points where is_validated = true, final weighting is not null, and validation weighting is either null or a parameterised value.
Remove "mustHaveFinalWeighting" parameter, because actually final weighting must always be non-null.
Improve code layout
Merge pull request #143 from SEEG-Oxford/203-amend-extent-generation

Amend disease extent generation to support disease group set-up