Add NWB export pytest to verify LFP data is the same for both export functions

Currently there are two methods to write LFP data to the NWB file (`add_ephys_lfp_from_source_to_nwb`, `add_ephys_lfp_from_dj_to_nwb`).  It would be good to have a pytest to ensure that the LFP data read from the NWB file is the same.