Algo: Add virtual destructors when needed.

That solves ``deleting object of polymorphic class type * which has
non-virtual destructor'' GCC warnings.

algo/core/affectanalyser.h

@@ -43,6 +43,8 @@ template<class T>
 class AffectAnalyser {
  public:
 
+  virtual ~AffectAnalyser() {}
+
   /* Queries */
 
   /**

algo/core/read_score.cpp

@@ -60,6 +60,7 @@ void KmerAffectReadScore::setUnknownScore(float score) {
 ////////////////////////////////////////////////////////////////////////////////
 
 ReadLengthScore::ReadLengthScore(){}
+ReadLengthScore::~ReadLengthScore(){}
 
 float ReadLengthScore::getScore(const string &sequence) const {
   return sequence.size();

algo/core/read_score.h

@@ -11,6 +11,8 @@
  */
 class VirtualReadScore {
  public:
+  virtual ~VirtualReadScore() {}
+
   /**
    * @param sequence: some text
    * @return the score associated to the sequence.
@@ -62,6 +64,7 @@ public:
 class ReadLengthScore: public VirtualReadScore {
  public:
   ReadLengthScore();
+  ~ReadLengthScore();
 
   /**
    * @return the sequence length