remove relabels and set_labels

[manuschneider]

Replaces the deprecated use of relabels by relabel, and the use of relabels_ and set_labels by relabel_.

[codecov]

Codecov Report

❌ Patch coverage is 41.66667% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.18%. Comparing base (e87c3a2) to head (2f2969f).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/tn_algo/DMRG.cpp	0.00%	12 Missing ⚠️
src/linalg/Lanczos_Exp.cpp	41.17%	10 Missing ⚠️
src/tn_algo/RegularMPS.cpp	0.00%	6 Missing ⚠️
src/linalg/Qdr.cpp	0.00%	3 Missing ⚠️
src/linalg/Qr.cpp	0.00%	2 Missing ⚠️
src/ncon.cpp	0.00%	0 Missing and 1 partial ⚠️
src/tn_algo/iMPS.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #729      +/-   ##
==========================================
+ Coverage   32.35%   35.18%   +2.83%     
==========================================
  Files         215      215              
  Lines       36363    33059    -3304     
  Branches    14597    13183    -1414     
==========================================
- Hits        11764    11633     -131     
+ Misses      22659    19513    -3146     
+ Partials     1940     1913      -27     

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