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[bot]]

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 :warning:
renormalizer/mps/symbolic_mpo.py	98.52%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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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