improve coverage by changing pytest-cov settings

[codecov-io]

Codecov Report

Merging #85 into master will decrease coverage by 7.93%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
- Coverage   75.02%   67.09%   -7.94%     
==========================================
  Files          24       14      -10     
  Lines         949      699     -250     
==========================================
- Hits          712      469     -243     
+ Misses        237      230       -7

Impacted Files	Coverage Δ
tests/test_spike.py
tests/test_epoch.py
tests/test_event.py
tests/test_nwb.py
tests/test_tensor.py
tests/test_traces.py
tests/test_utils.py
tests/test_calcium.py
setup.py
tests/test_deconvolution.py

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[mochic]

it's a decrease in coverage but a more realistic representation of what our actual coverage is