Create StandardMeanError.java

[RubyflameWarrior]

• [x] I have read CONTRIBUTING.md.
• [X] This pull request is all my own work -- I have not plagiarized it.
• [X] All filenames are in PascalCase.
• [X] All functions and variable names follow Java naming conventions.
• [X] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [X] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[RubyflameWarrior]

Please merge this branch using the "hacktoberfest-accepted" label.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[github-actions[bot]]

Please reopen this pull request once you commit the changes requested or make improvements on the code. If this is not the case and you need some help, feel free to seek help from our Gitter or ping one of the reviewers. Thank you for your contributions!