cleanup: Change Salty to Brackish

In the code, the difference between SALTYWATER and SALTWATER is hard
to see. More importantly, in the UI - Brackish is the word for water
that has more salt that freshwater but less salt that seawater. The
docs already use the word to clarify what is meant.

core/units.h

@@ -32,6 +32,7 @@ extern "C" {
 /* Salinity is expressed in weight in grams per 10l */
 #define SEAWATER_SALINITY 10300
 #define EN13319_SALINITY 10200
+#define BRACKISH_SALINITY 10100
 #define FRESHWATER_SALINITY 10000
 
 #include <stdint.h>