Account for fact that background level now coded in Limit via use of same EMS code

ie

Colour Apparent | Total | EMS_0001 | Aquatic Life - Freshwater | Water | Long-term chronic | 30 | 5 | mean |   |   |   |   | No Effect | Lower Limit | EMS_0001 * 0.8 | white light
-- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | --

No file or test is named. Start by locating the handling of the Limit field and EMS_0001 in the R package's water-quality threshold and index calculation paths. Done means the background level is correctly represented when it uses the same EMS code.

Written by the indexing model from the issue text.