combined NWB embedded metadata

We would like to add metadata to combined nwbs that we make in #6 .
Here's what we have decided for now. Please check if anything else need to be updated. @tmchartrand @arjunsridhar12345 @arielleleon :

- General metadata can be inherited from nwb's source data assets and can stay in `lab metadata `field.

- Additional metadata specific to nwb (the code that packed it) should be added in `source_script ` field.

Sources of general metadata:

- Inherited metadata from derived session-level behavior and physiology data. (See #10 )

- Inherited metadata from raw and processed pose tracking data. (See #)

- Any addtional processing for fields specific for nwb.