[WIP] Imaginary time TEBD for MPSs.

[AidanGG]

For #55. For now I just extended the __init__ function with an imag param. I added a basic test showing it with ZZ interactions.

[pep8speaks]

Hello @AidanGG! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file tests/test_tensor/test_tensor_tebd.py:

Line 102:80: E501 line too long (84 > 79 characters) Line 103:80: E501 line too long (81 > 79 characters)

Comment last updated at 2020-05-03 13:50:53 UTC

[codecov[bot]]

Codecov Report

Merging #57 into develop will decrease coverage by 1.09%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #57      +/-   ##
===========================================
- Coverage    87.32%   86.23%   -1.10%     
===========================================
  Files           32       32              
  Lines         8674     8684      +10     
===========================================
- Hits          7575     7489      -86     
- Misses        1099     1195      +96     

Impacted Files	Coverage Δ
quimb/tensor/tensor_tebd.py	89.53% <100.00%> (+0.36%)	:arrow_up:
quimb/tensor/optimize_autograd.py	37.17% <0.00%> (-50.27%)	:arrow_down:
quimb/tensor/tensor_1d.py	83.85% <0.00%> (+0.02%)	:arrow_up:

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[jcmgray]

Looks good to me! Failing tests unrelated. Might be interesting to see how this compares to the 2D simple update algorithm I've just implemented, (with Ly=1).