Deal with duplicate variables

[joethorley]

# A tibble: 8 x 4
  Variable              Code     Units EC_Code
  <chr>                 <chr>    <chr>   <int>
1 Chromium Total        EMS_CR_T ug/L      463
2 Chromium Total        EMS_CR_T mg/L      463
3 Conductivity Specific EMS_SC_F mS/cm      NA
4 Conductivity Specific EMS_0011 mS/cm      NA
5 Temperature           EMS_0013 degC      840
6 Temperature           EMS_TEMF degC       NA
7 Turbidity             EMS_TURF NTU        NA
8 Turbidity             EMS_0015 NTU       864

In particular set Chromium to have the units in the limits table.

Distinguish between Temperature and Temperature - Field (TEMF)
Treat Conductivity and Turbidity as the same with the same EC_Code

[joethorley]

Currently I've given them all unique Variable names (as well as codes)