Fix bug in DMRG-X with QNs

[mtfishman]

Fixes #45.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (161209a) 86.50% compared to head (5e2ffa2) 86.50%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #61   +/-   ##
=======================================
  Coverage   86.50%   86.50%           
=======================================
  Files          17       17           
  Lines         600      600           
=======================================
  Hits          519      519           
  Misses         81       81           

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