fix #251

[d-chambers]

Description

This PR address issue #251. It also fixed a performance bug where slicing a DirectorySpool could result in loading patches into memory prematurely.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings or appropriate doc page.
• included a test. See testing guidelines.
• your name has been added to the contributors page (docs/contributors.md).
• added the "ready_for_review" tag once the PR is ready to be reviewed.

[codecov]

Codecov Report

Merging #256 (78bb4a0) into master (453deee) will decrease coverage by 0.01%.
Report is 2 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
- Coverage   99.06%   99.06%   -0.01%     
==========================================
  Files          79       79              
  Lines        6440     6438       -2     
==========================================
- Hits         6380     6378       -2     
  Misses         60       60              

Flag	Coverage Δ
unittests	99.06% <100.00%> (-0.01%)	⬇️

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Files Changed	Coverage Δ
dascore/clients/dirspool.py	100.00% <100.00%> (ø)
dascore/core/spool.py	100.00% <100.00%> (ø)