cli: download-files - Add --dry-run and --verify

[ParthS007]

-> cli: download-files - Add --dry-run

• Added --dry-run for listing file locations
• Added tests for --dry-run command
• Added feature for multiple filters for the same type

-> cli: download-files add --verify option

• Added verify option to verify the files after the download
• Added a new function for checking files
• Added docstrings wherever possible.
• Added tests for newly created functions.

closes #48

[ParthS007]

@tiborsimko It is ready for review. :)

[ParthS007]

Again ready for review :)