SU(2) Exact Diagonalization

[qsukhi]

Added a function to compute the low energy eigenstates of a local, SU(2)-symmetric Hamiltonian, in a way that exploits the SU(2) symmetry. Also added some generic SU(2) functionality e.g. functions to compute Clebsch-Gordan coefficients, Fusion Rules, Fusing two indices etc.

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[qsukhi]

@googlebot I signed it!

[mganahl]

thanks @qsukhi I'll take a look!

[mganahl]

Hi @qsukhi, I'm working through the code. It will take me a bit of time since it is a lot of code. In the meantime, can you change indenting to two spaces instead of four? I wonder why the linter didn't catch this.
Also, I noticed that there is a binary file in this PR, is this intentional?

[qsukhi]

Thanks, @mganahl.
-I've reduced the indent to 2 spaces.
-The binary file is a table of factors that come from the Clebsch-Gordan part of SU(2)-symmetric tensor contractions. It is not necessary, as the function Su2ExactDiagonalization() can build the factor tables if the user specifies so (as illustrated in Su2ExactDiagonalization/main.py).

[codecov-io]

Codecov Report

Merging #898 (e1be4b0) into master (523e7a8) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #898      +/-   ##
==========================================
- Coverage   98.12%   98.08%   -0.05%     
==========================================
  Files         129      129              
  Lines       22790    22257     -533     
==========================================
- Hits        22362    21830     -532     
+ Misses        428      427       -1     

Impacted Files	Coverage Δ
...ornetwork/matrixproductstates/infinite_mps_test.py	84.78% <0.00%> (-1.50%)	⬇️
...ckends/tensorflow/tensorflow_tensornetwork_test.py	59.75% <0.00%> (-1.17%)	⬇️
tensornetwork/linalg/tests/initialization_test.py	84.61% <0.00%> (-0.65%)	⬇️
tensornetwork/backends/decorators_test.py	92.64% <0.00%> (-0.51%)	⬇️
...ensornetwork/tests/tensornetwork_symmetric_test.py	92.70% <0.00%> (-0.37%)	⬇️
tensornetwork/backends/decorators.py	96.77% <0.00%> (-0.20%)	⬇️
tensornetwork/linalg/tests/test_linalg.py	96.31% <0.00%> (-0.20%)	⬇️
tensornetwork/tests/tensor_test.py	98.51% <0.00%> (-0.15%)	⬇️
examples/simple_mera/simple_mera.py	95.49% <0.00%> (-0.12%)	⬇️
...tractors/opt_einsum_paths/path_calculation_test.py	95.74% <0.00%> (-0.09%)	⬇️
... and 44 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 523e7a8...e1be4b0. Read the comment docs.