Make fill proportion a parameter to gen()

sud.h

@@ -41,9 +41,10 @@ enum update_res { NOT_ALLOWED, ALREADY_DET, OK };
 enum check_res { INCOMPLETE, INCORRECT, SOLVED };
 
 /**
- * Populate the sudoku with some random values.
+ * Populate the sudoku with some random values. The proportion of
+ * cells that are filled will be approximately `fill_prop`.
  */
-void gen(struct sudoku *sud);
+void gen(struct sudoku *sud, double fill_prop);
 
 /**
  * Attempt to update the cell at row `r`, column `c` to have the value