Add hash function for two doubles (for non-polarized Fresnel)

Core/Multilayer/HashKVector.cpp

@@ -20,3 +20,8 @@ size_t HashKVector::operator()(kvector_t kvec) const noexcept
 {
     return m_double_hash(kvec.x()) ^ m_double_hash(kvec.y()) ^ m_double_hash(kvec.z());
 }
+
+size_t Hash2Doubles::operator()(std::pair<double, double> doubles) const noexcept
+{
+    return m_double_hash(doubles.first) ^ m_double_hash(doubles.second);
+}

Core/Multilayer/HashKVector.h

@@ -18,6 +18,7 @@
 
 #include "Vectors3D.h"
 #include <functional>
+#include <utility>
 
 class HashKVector
 {
@@ -30,4 +31,15 @@ private:
     std::hash<double> m_double_hash;
 };
 
+class Hash2Doubles
+{
+public:
+    Hash2Doubles() {}
+    ~Hash2Doubles() {}
+
+    size_t operator()(std::pair<double, double> doubles) const noexcept;
+private:
+    std::hash<double> m_double_hash;
+};
+
 #endif // HASHKVECTOR_H