Add power function and test to backend

[wkostuch]

First step as proposed in #840 to add support for the np.power function.

[alewis]

Before code can be merged, it needs to "build", which means for example that pylint and pytest have to run successfully. You can tell whether your code has built properly by looking at the line marked "continuous-integration/travis-ci/pr", which will have a red X / green checkmark upon failure/success. The same symbols appear next to the PR in the PR list.

If the code doesn't build, you can click Details on the same line to see which configurations have failed, and then click on any one of them to see the console output from each. In your case the error is quite simple: line 1001 in abstract_backend.py exceeds the maximum character limit of 80 characters.

[alewis]

Looks good mostly! Fix the small formatting issues here and I'll merge.

[alewis]

Ok, looks good. Fix the build and I'll merge.

[codecov-commenter]

Codecov Report

Merging #846 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #846   +/-   ##
=======================================
  Coverage   98.37%   98.37%           
=======================================
  Files         129      129           
  Lines       22103    22109    +6     
=======================================
+ Hits        21743    21749    +6     
  Misses        360      360           

Impacted Files	Coverage Δ
tensornetwork/backends/abstract_backend.py	92.30% <100.00%> (+0.09%)	⬆️
tensornetwork/backends/backend_test.py	100.00% <100.00%> (ø)

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