changed get() and slicing

[manuschneider]

UniTensors keep their metadata (possibly with bonds removed for constant access of one element in a given dimension).
Diagonal UniTensors can be accessed by two-dimensional slicing as well now.

This fixes #397

[codecov]

Codecov Report

❌ Patch coverage is 75.32468% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.55%. Comparing base (fa015d4) to head (5b89af3).
⚠️ Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
src/DenseUniTensor.cpp	65.85%	7 Missing and 7 partials ⚠️
include/UniTensor.hpp	58.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #714      +/-   ##
==========================================
+ Coverage   35.44%   35.55%   +0.10%     
==========================================
  Files         215      214       -1     
  Lines       33071    33118      +47     
  Branches    13170    13191      +21     
==========================================
+ Hits        11723    11776      +53     
+ Misses      19424    19412      -12     
- Partials     1924     1930       +6     

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[hunghaoti]

Is it possible to add some simple unit test?

[ianmccul]

@codex review

[chatgpt-codex-connector]

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[manuschneider]

All issues were fixed, and unit tests were added.

I changed the behavior so that slicing a diagonal-dense UniTensor with two accessors always yields a non-diagonal UniTensor. Before, the result was still diagonal if the two accessors were the same, but this led to inconsistent behavior. If the output should be diagonal, use only one accessor!