Change small metadata datasets to attributes where appropriate

[oruebel]

Change Change small metadata datasets to attributes where appropriate:

Reason: Storing small metadata as datasets (compared to attributes) can lead to: i) clutter in the file hierarchy making it harder for users to navigate files, ii) makes metadata appear as core data, and iii) causes poor performance when extracting metadata from files (reading attributes is more efficient in many cases).

Select Specific Changes Change top-level datasets identifier, nwb_version, session_description, session_start_time from datasets to attributes. @t-b suggests in #45 that the top-level dataset file_create_date should remain a dataset as this datasets may need to be update/extended repeatedly rather than just being just a a static piece of metadata.