KMerAffect: Use AFFECT_UNKNOWN_SYMBOL and AFFECT_AMBIGUOUS_SYMBOL constants

algo/core/kmeraffect.cpp

@@ -50,9 +50,9 @@ bool operator!=(const affect_t &a1, const affect_t &a2) {
 string toString(const affect_t &a) {
   string result;
   if(a ==AFFECT_UNKNOWN)
-    result = " _";
+    result = " "AFFECT_UNKNOWN_SYMBOL;
   else if (a == AFFECT_AMBIGUOUS)
-    result = " ?"; 
+    result = " "AFFECT_AMBIGUOUS_SYMBOL; 
   else
     result = (affect_strand(a)==1 ? "+" : "-") + string(1,affect_char(a));
   return result;