Attach characteristics and ddi reader bug fix

[renae-r]

This release corrects two bugs:

1. MTUS and AHTUS collections do not support attach characteristics; this release ensures that their extract definitions do not contain empty attachCharacteristics elements.

2. A change to DDI files for hierarchical extracts is now accomodated by the readers.read_ipums_ddi()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.37%. Comparing base (db6e7e5) to head (d1f4e45).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
+ Coverage   88.20%   88.37%   +0.17%     
==========================================
  Files          14       14              
  Lines        1365     1368       +3     
==========================================
+ Hits         1204     1209       +5     
+ Misses        161      159       -2     

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