add possibility to iterate over SparseVector data at runtime (#143)

matrix/SparseVector.hpp

@@ -60,7 +60,7 @@ private:
     }
 
 public:
-    static constexpr size_t non_zeros() {
+    constexpr size_t non_zeros() const {
         return N;
     }
 
@@ -94,6 +94,16 @@ public:
         return _data[compressed_index];
     }
 
+    inline Type atCompressedIndex(size_t i) const {
+        assert(i < N);
+        return _data[i];
+    }
+
+    inline Type& atCompressedIndex(size_t i) {
+        assert(i < N);
+        return _data[i];
+    }
+
     void setZero() {
         for (size_t i = 0; i < N; i++) {
             _data[i] = Type(0);