NWB Export: Include animal strain and source in Subject

[rly]

Ideally there would be zero loss of information in the process of exporting to NWB. Animal strain and source are part of the Subject table in DataJoint but not stored in the NWB subject. Although NWB does not have a structured field for those data, please add them into the subject's description.

https://github.com/vathes/DJ-NWB-Li-Daie-2015-2016/blob/ae69335a23e6a3c0c3809a54de5480baf8848712/pipeline/export/datajoint_to_nwb.py#L65-L71