Change output fields structure to PartType%i

examples/output_fields_example.yml

-OutputFields:
+# Gas
+PartType0:
+  Coordinates: 1 # Request to write output
+  Masses: 1
+  Velocities: 1
+  SmoothingLenght: 1
+  Entropy: 1
+  ParticleIDs: 1
+  Density: 1
+  InternalEnergy: 0 # Request to not write output
+  Pressure: 0
+  Potential: 0
+
+# Dark Matter
+PartType1:
   Coordinates: 1
-  Masses: 0
-  SmoothingLenght: 1
\ No newline at end of file
+  Velocities: 1
+  Masses: 1
+  ParticleIDs: 1
+  Potential: 0
+
+# Stars
+PartType2:
+  Coordinates: 1
+  Velocities: 1
+  Masses: 1
+  ParticleIDs: 1
\ No newline at end of file

src/single_io.c

@@ -827,8 +827,8 @@ void write_output_single(struct engine* e, const char* baseName,
 
     /* Write everything */
     for (int i = 0; i < num_fields; ++i) {
-      char field[200] = "OutputFields:";
-      strcat(field, list[i].name);
+      char field[200];
+      sprintf(field, "ParticleType%i:%s", i, list[i].name);
       int should_write = parser_get_opt_param_int(output_fields, field,
                                                   list[i].default_output);
       if (should_write)