Define new macros for K-mer Ambiguous and Unknown

Since these two K-mers need to be printed with a different char, this char
is defined as a new macro used exclusively to represent the K-mer in a string.

For more informations see #3250.

algo/core/kmeraffect.h

@@ -175,6 +175,11 @@ ostream &operator<<(ostream &os, const KmerAffect &kmer);
 #define AFFECT_AMBIGUOUS_CHAR (AFFECT_AMBIGUOUS_SYMBOL[0])
 #define AFFECT_UNKNOWN_CHAR (AFFECT_UNKNOWN_SYMBOL[0])
 
+/* Define how an ambiguous kmeraffect looks like in a string */
+#define AFFECT_AMBIGUOUS_TO_STRING "?"
+
+/* Define how an unknown kmeraffect looks like in a string */
+#define AFFECT_UNKNOWN_TO_STRING "_"
 
 /**
  * Constant defining any not-unknown affectation