Add to_json implementation

[aamster]

The current workflow for writing NWB is to read json (generated from LIMS), then write to NWB

A new approach which we previously discussed is to let AllenSDK create the snapshots without the LIMS (ruby generating JSON code) requirement.

The new workflow would be from_internal -> to_json, to_nwb.

This ticket is for implementing to_json in DataObject.

Thoughts: do we want the new json to be exactly the same as the current json? We now have flexibility to write it however we want.

• to_json implemented for all DataObject