NWBFile properties not being saved

[rly]

Properties of NWBFile are not being saved to disk.
We also need a roundtrip test on this.

from pynwb import NWBFile, NWBHDF5IO
from datetime import datetime

nwbfile = NWBFile(session_description='ADDME',
                  identifier='ADDME',
                  session_start_time=datetime.now().astimezone(),
                  experimenter=('ADDME1', 'ADDME2'),
                  related_publications='ADDME',
                  institution='ADDME')

with NWBHDF5IO('test_file.nwb', 'w') as io:
    io.write(nwbfile)

with NWBHDF5IO('test_file.nwb', 'r') as io:
    read_nwbfile = io.read()
    print(read_nwbfile)
    print(read_nwbfile.experimenter)
    print(read_nwbfile.related_publications)
    print(read_nwbfile.institution)
    # these are also not present when inspecting the file using HDFView

[rly]

This is due to the removal of automatic mapping of NWBFile group "general" dataset x to NWBFile.x.
hdmf-dev/hdmf#130