Updated website after changes to Grouping calculated property names

Sum => sum
Total => total
Average => mean
Count => count
Updated docs for Dimension and Grouping
Updated change log
Updated website after Grouping documentation updates
Fixed mistakes in change log
Replaced total with totalCount in result of counting Grouping properties
Merge pull request #261 from niksoper/gh-pages-dev-review-groupings

Updated website after review of groupings
Merge branch 'gh-pages-dev' of https://github.com/ScottLogic/insight into feature/add-theme-demo