Add epanet input schema

sql/section_create/20.mixing.sql

@@ -0,0 +1,10 @@
+-- [MIXING]
+
+CREATE TYPE MIXING_Model AS ENUM ('MIXED', '2COMP', 'FIFO', 'LIFO');
+
+CREATE TABLE MIXING
+(
+  Tank  VARCHAR(32) PRIMARY KEY REFERENCES TANKS(ID)
+, Model MIXING_Model NOT NULL
+, Value NUMERIC
+);