Replace getLabel return statement by toStringValues call

Since the methods returns the same thing, the getLabel instructions can simply
return the toStringValues function result.

For more informations see #3250.

algo/core/kmeraffect.cpp

@@ -155,7 +155,7 @@ int KmerAffect::getStrand() const{
 }
 
 string KmerAffect::getLabel() const {
-  return string(1, affect_char(affect));
+ return ::toStringValues(affect);
 }
 
 unsigned char KmerAffect::getLength() const {