Add functionality in mp/mps/mpo and add ExpDVR basis

[jjren]

1. support the addition of many mpos/mpss together.
   mps.add([mps1, mps2,....])
2. convertion between diagonal mpo and mps.
   Mpo.from_mps() Mps.from_mpo()
3. ExpDVR basis
4. add shift algorithm for excited states calculation ($H=H_0 + \sum_i \alpha_i |\psi_i><\psi_i|$).

[codecov]

Codecov Report

Patch coverage: 66.66% and project coverage change: -0.50% ⚠️

Comparison is base (9583324) 84.79% compared to head (2236e71) 84.30%.
Report is 1 commits behind head on master.

❗ Current head 2236e71 differs from pull request most recent head d49fdae. Consider uploading reports for the commit d49fdae to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
- Coverage   84.79%   84.30%   -0.50%     
==========================================
  Files         105      105              
  Lines       10183    10355     +172     
==========================================
+ Hits         8635     8730      +95     
- Misses       1548     1625      +77     

Files Changed	Coverage Δ
renormalizer/mps/mps.py	89.21% <27.58%> (-1.31%)	⬇️
renormalizer/model/basis.py	80.81% <40.19%> (-7.96%)	⬇️
renormalizer/mps/mpo.py	90.56% <84.61%> (-0.60%)	⬇️
renormalizer/mps/mp.py	85.16% <95.00%> (+0.40%)	⬆️
renormalizer/mps/gs.py	96.42% <100.00%> (+0.49%)	⬆️
renormalizer/mps/mpdm.py	88.46% <100.00%> (-1.65%)	⬇️
renormalizer/mps/tests/test_gs.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[atxy-blip]

Looks good to me : )

[liwt31]

The test coverage is too low currently - it seems easy to add a few tests for these new functions

[jjren]

I will add doc and tests later.

[jjren]

I close the PR now and will open it again when the new features for nuclear dynamics are ready.