Fix low flow sediment value calculation
Fix low flow sediment value calculation

Sediment is in tonnes, so it first needs be converted from tonnes to
kilograms before being converted from kilograms to milligrams.

This change is based on the sample calculations in
https://drive.google.com/a/azavea.com/file/d/0B0hs2mX4ApaLYXBPS2RwTG1JZ1FOZ2UybXpBTEdaU2VaWVdz/view,
as well as Barry's comments in https://github.com/WikiWatershed/model-my-watershed/issues/1441.

Refs https://github.com/WikiWatershed/model-my-watershed/issues/1441

kdeloach/gwlf-e,WikiWatershed/gwlf-e