Refactored some files to remove code smells.

[Rushipatel0995]

Describe your change:

• Refactored some files for the algorithms. Since this is a refactoring change I have merged the refactoring for Hill Cipher, SinglyLinkedList and DoublyLinkedList.
• Removed duplicate codes by extracting method, renamed variables for better understandability, pulled up variables/methods for better consistency, extracted class to remove feature envy smell and moved method for the same functionality classes. If the pull request is accepted I am looking forward to refactor this entire project since I found multiple smells on SonarQube as well as in DesigniteJava.

References

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Java files are placed inside an existing directory.
• All filenames are in all uppercase characters with no spaces or dashes.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.