Make Grid iterable

lib/src/Grid.cpp

@@ -0,0 +1,19 @@
+#include "aocpp/Grid.hpp"
+
+namespace aocpp {
+auto GridIterator::operator*() -> GridElement
+{
+    return {{x_, y_}, grid_[Coord{x_, y_}]};
+}
+
+auto GridIterator::operator++() -> GridIterator &
+{
+    if (++x_ == grid_.columns)
+    {
+        x_ = 0;
+        y_++;
+    }
+
+    return *this;
+}
+} // namespace