Added cache_spec==True keyword to lfp_writer.write call

[pickles-bread-and-butter]

NWB advises to include this keyword in the NWB file write. This keyword caches the extension in the NWB file so it's easier for others to read data. No need to import ecephys.nwb.

[codecov-io]

Codecov Report

Merging #1427 into rc/1.6.0 will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           rc/1.6.0    #1427      +/-   ##
============================================
+ Coverage     34.59%   34.71%   +0.11%     
============================================
  Files           342      342              
  Lines         33243    33253      +10     
============================================
+ Hits          11501    11544      +43     
+ Misses        21742    21709      -33     

Impacted Files	Coverage Δ
...dk/brain_observatory/ecephys/write_nwb/__main__.py	67.59% <100.00%> (+0.23%)	⬆️
allensdk/internal/api/behavior_data_lims_api.py	52.84% <0.00%> (-1.55%)	⬇️
...ephys/stimulus_analysis/receptive_field_mapping.py	89.01% <0.00%> (-0.44%)	⬇️
...dk/brain_observatory/behavior/trials_processing.py	24.46% <0.00%> (-0.07%)	⬇️
allensdk/internal/api/behavior_ophys_api.py	28.93% <0.00%> (+0.21%)	⬆️
...brain_observatory/ecephys/copy_utility/__main__.py	63.52% <0.00%> (+0.43%)	⬆️
allensdk/internal/brain_observatory/time_sync.py	24.84% <0.00%> (+24.84%)	⬆️

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