change conversion from string to arithmetic

example/parser.example.cpp

@@ -0,0 +1,26 @@
+#include <Modelec/CLParser.h>
+#include <iostream>
+
+struct B {
+    std::string a;
+};
+
+int main(int argc, char* argv[]) {
+
+    CLParser parser(argc, argv);
+
+    int port = parser.getOption("port", 12);
+
+    std::cout << "Port : " << port << std::endl;
+
+    auto test = parser.getOption<unsigned long long>("long");
+
+    if (test.has_value()) {
+        std::cout << test.value() << std::endl;
+    }
+    else {
+        std::cout << "missing long argument" << std::endl;
+    }
+
+    return 0;
+}