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 21.81818% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.95%. Comparing base (eb3e3be) to head (7cc7ba7).
⚠️ Report is 67 commits behind head on master.

Files with missing lines	Patch %	Lines
include/UniTensor.hpp	10.52%	34 Missing ⚠️
src/Accessor.cpp	0.00%	5 Missing ⚠️
src/backend/Tensor_impl.cpp	60.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #714      +/-   ##
==========================================
+ Coverage   30.19%   31.95%   +1.75%     
==========================================
  Files         215      215              
  Lines       38335    36227    -2108     
  Branches    15474    14538     -936     
==========================================
+ Hits        11576    11577       +1     
+ Misses      24858    22749    -2109     
  Partials     1901     1901              

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[hunghaoti]

Is it possible to add some simple unit test?