added [[deprecated]] marker and updated docstrings for deprecations

[manuschneider]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.25%. Comparing base (e87c3a2) to head (4080739).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
include/UniTensor.hpp	0.00%	3 Missing ⚠️
include/backend/Storage.hpp	0.00%	2 Missing ⚠️
include/backend/Scalar.hpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #748      +/-   ##
==========================================
+ Coverage   32.35%   35.25%   +2.90%     
==========================================
  Files         215      215              
  Lines       36363    33095    -3268     
  Branches    14597    13196    -1401     
==========================================
- Hits        11764    11669      -95     
+ Misses      22659    19512    -3147     
+ Partials     1940     1914      -26     

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