Add #include <string> for std::stod

See https://en.cppreference.com/w/cpp/string/basic_string/stof for details.

matplotlibcpp.h

@@ -13,6 +13,7 @@
 #include <iostream>
 #include <cstdint> // <cstdint> requires c++11 support
 #include <functional>
+#include <string> // std::stod
 
 #ifndef WITHOUT_NUMPY
 #  define NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION