adding SubTree model

Author: facontidavide

include/behaviortree_cpp/basic_types.h

@@ -158,13 +158,16 @@ inline StringConverter GetAnyFromStringFunctor<void>()
 template<typename T> [[nodiscard]]
 std::string toStr(const T& value)
 {
-  if constexpr(!std::is_arithmetic_v<T>)
+  if constexpr (std::is_convertible_v<T, std::string> or
+                std::is_convertible_v<T, std::string_view>)
+  {
+    return value;
+  }
+  else if constexpr(!std::is_arithmetic_v<T>)
   {
     throw LogicError(
         StrCat("Function BT::toStr<T>() not specialized for type [",
-               BT::demangle(typeid(T)), "],",
-               "Implement it consistently with BT::convertFromString<T>(), "
-               "or provide at dummy version that returns an empty string.")
+               BT::demangle(typeid(T)), "]")
       );
   } else {
     return std::to_string(value);

include/behaviortree_cpp/scripting/operators.hpp

@@ -380,6 +380,16 @@ struct ExprComparison : ExprBase
           return False;
         }
       }
+      else if ((lhs_v.isString() && rhs_v.isNumber()) ||
+               (lhs_v.isNumber() && rhs_v.isString()))
+      {
+        auto lv = lhs_v.cast<double>();
+        auto rv = lhs_v.cast<double>();
+        if (!SwitchImpl(lv, rv, ops[i]))
+        {
+          return False;
+        }
+      }
       else
       {
         throw RuntimeError(StrCat("Can't mix different types in Comparison. "