Added mid_time() to FileMetadata issue #53

[safimuhammad]

• mid_time() is now moved to FileMetadata class, calculated mid-range of file metadata.
• Test case for FileMetadata.mid_time() also added in test_file_metadata.py.

ps: Thanks for the help, its a great project.

[safimuhammad]

• changed the branch from main to develop.
• removed validations from file_metadata.py.
• updated test_file_mid_time(), added a constant value instead of a formula.