[BL] Use std::string::replace instead of substr +

This is more efficient.

BaseLib/FileTools.cpp

@@ -111,9 +111,10 @@ bool substituteKeyword(std::string& result, std::string& parenthesized_string,
     std::string const generated_fmt_string =
         "{" + precision_specification +
         type_specification[std::type_index(typeid(data))] + "}";
-    result = result.substr(0, begin) +
-             fmt::vformat(generated_fmt_string, fmt::make_format_args(data)) +
-             result.substr(end + 1, result.length() - (end + 1));
+    result.replace(
+        begin, end - begin + 1,
+        fmt::vformat(generated_fmt_string, fmt::make_format_args(data)));
+
     return true;
 }