Updating MySQL distance calculations to use metric (#921)

MySQL geo distace calculation functions currently return results in miles
instead of kms. Multipliying result of function by 1.609344 should fix the
issue.

Dev Note: Tested with a few situations with the report search and report
view pages, which seems to yield the correct results. However, I'm not
entirely confident I was able to explore all use cases for the
Incident::get_incidents() and Incident::get_neighbouring_incidents().
Adding a bit of doco around maths
Merge pull request #1241 from ditorelo/921

Updating MySQL distance calculations to use metric (#921)