Allow generators to declare themselves (infinite)

This will be useful later to implement warning on infinitely
running `GENERATE` expressions.

examples/300-Gen-OwnGenerator.cpp

@@ -40,6 +40,8 @@ public:
         return true;
     }
 
+    bool isFinite() const override { return false; }
+
     // Note: this improves the performance only a bit, but it is here
     //       to show how you can override the skip functionality.
     void skipToNthElementImpl( std::size_t n ) override {

examples/301-Gen-MapTypeConversion.cpp

@@ -40,6 +40,8 @@ public:
     bool next() override {
         return !!std::getline(m_stream, m_line);
     }
+
+    bool isFinite() const override { return true; }
 };
 
 std::string const& LineGenerator::get() const {