fixes a bug in contract when the two fermionic tensors have no common labels

[manuschneider]

Fixes #1033
Also minor fixes of comments, and removal of an unused variable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.89%. Comparing base (f5b33cf) to head (2a9ecfd).
⚠️ Report is 9 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1186   +/-   ##
=======================================
  Coverage   68.89%   68.89%           
=======================================
  Files         208      208           
  Lines       22411    22406    -5     
  Branches       72       72           
=======================================
- Hits        15439    15437    -2     
+ Misses       6950     6947    -3     
  Partials       22       22           

Flag	Coverage Δ
cpp	68.98% <100.00%> (+<0.01%)	⬆️
python	63.96% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
C++ backend	68.98% <100.00%> (+<0.01%)	⬆️
Python bindings	∅ <ø> (∅)
Python package	63.96% <ø> (ø)

Files with missing lines	Coverage Δ
include/Symmetry.hpp	7.01% <ø> (ø)
src/BlockFermionicUniTensor.cpp	65.65% <100.00%> (-0.06%)	⬇️
src/Bond.cpp	65.96% <100.00%> (-0.31%)	⬇️

... and 4 files with indirect coverage changes

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