Use QR for MPO construction and allow calculating entropies with TTN

[liwt31]

• By default, use QR instead of Hopcroft-Carp for MPO construction, which in some cases will produce a more compact MPO and the time cost is comparable.
  • Currently using dense QR. Opt to sparse QR could potentially improve the performance
  • The Hopcroft-Carp algorithm is still accessible by setting the algo argument.
  • The local MPO by QR is numerically different from the Hopcroft-Carp algorithm and is generally more dense.
• Allow TTNS to calculation 1/2-site RDM/entropy, 1/2-dof RDM/entropy and mutual entropy.
• Also improved README a bit.

[codecov]

Codecov Report

Attention: Patch coverage is 95.84055% with 24 lines in your changes missing coverage. Please review.

Project coverage is 86.21%. Comparing base (cd1f4ed) to head (083abf2).

Files	Patch %	Lines
renormalizer/tn/tree.py	93.80%	23 Missing ⚠️
renormalizer/mps/symbolic_mpo.py	98.52%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage   85.98%   86.21%   +0.22%     
==========================================
  Files         118      118              
  Lines       12494    12779     +285     
==========================================
+ Hits        10743    11017     +274     
- Misses       1751     1762      +11     

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