Expose cache_spec option in NWBHDF5IO.export

[rly]

Motivation

Right now, NWBHDF5IO.export always caches the spec. The option to change that is not exposed. This PR exposes that keyword argument. Ref: hdmf-dev/hdmf#1187

Checklist

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[rly]

I don't think tests are necessary for this quick change, but I could write one if you think we should have it @stephprince

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.20%. Comparing base (71ef8e2) to head (124332d).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1959   +/-   ##
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  Coverage   92.20%   92.20%           
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  Files          27       27           
  Lines        2656     2656           
  Branches      693      693           
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  Hits         2449     2449           
  Misses        134      134           
  Partials       73       73           

Flag	Coverage Δ
integration	73.19% <ø> (ø)
unit	83.77% <ø> (ø)

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[stephprince]

Looks good to me! I think it is ok to not have tests for this small argument exposure.