Check blockidx out of range for dense UniTensor

[manuschneider]

Fixes #176 and #298

[codecov]

Codecov Report

❌ Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.76%. Comparing base (19643a1) to head (ebd890c).
⚠️ Report is 45 commits behind head on master.

Files with missing lines	Patch %	Lines
include/UniTensor.hpp	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #701      +/-   ##
==========================================
+ Coverage   32.74%   32.76%   +0.01%     
==========================================
  Files         215      215              
  Lines       35065    35079      +14     
  Branches    14070    14075       +5     
==========================================
+ Hits        11482    11492      +10     
- Misses      21708    21712       +4     
  Partials     1875     1875              

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