[ndnSIM] Bug Report and Fix: std::optional Not Recognized During ndnSIM Installation on Ubuntu 22.04

[Mohammad Reza Shakournia]

Dear ndnSIM Community,

	My name is Mohammad Reza Shakoutnia, and I am a PhD candidate in Computer Science at the University of Tehran, Iran. I hope this message finds you well. I am writing to report a bug I encountered while installing ndnSIM on Ubuntu 22.04, and to share a solution that might benefit others facing the same issue.

	Issue Description: During the installation process, after running the ./waf command, I received an error indicating that the std::optional template was not recognized. This error prevented the successful compilation and installation of ndnSIM.

	Solution Implemented: To resolve this issue, I added the following line at the beginning of the scheduler.hpp file located in the ns3/ndnSIM/ndn-cxx/util directory:

	#include <optional>

	This inclusion resolved the compilation error related to std::optional.

	Steps to Replicate:
	* Set up a fresh installation of Ubuntu 22.04.
	* Follow the standard installation procedure for ndnSIM.
	* Run the ./waf command and observe the mentioned error.
	Suggested Actions: It may be beneficial for the maintainers to include this header file in future releases to ensure compatibility with Ubuntu 22.04 environments, preventing similar issues for other users.

	I appreciate your attention to this matter and am happy to provide any further details if needed. Thank you for your ongoing support and development of ndnSIM.

	Best regards,
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Mohammad Reza ShakourniaPhD Candidate in Computer Science,Member of Router Laboratory,Electrical and Computer Engineering Department,University of Tehran.
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