Adds EN_getresultindex function to the API

See issue #546 . Also fixes a small bug in project.c.

src/epanet2.c

@@ -7,7 +7,7 @@
  Authors:      see AUTHORS
  Copyright:    see AUTHORS
  License:      see LICENSE
- Last Updated: 05/15/2019
+ Last Updated: 10/26/2019
  ******************************************************************************
 */
 
@@ -227,6 +227,12 @@ int DLLEXPORT ENgetstatistic(int type, EN_API_FLOAT_TYPE *value)
     return errcode;
 }
 
+int DLLEXPORT ENgetresultindex(int type, int index, int *value)
+{
+    return EN_getresultindex(_defaultProject, type, index, value);
+}
+
+
 /********************************************************************
 
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