#652 cache evaluation array

[valentinsulzer]

Description

Reuse evaluation array when making a copy of a state vector
Fixes #653

Type of change

• New feature (non-breaking change which adds functionality)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Merging #653 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #653      +/-   ##
==========================================
+ Coverage   98.49%   98.49%   +<.01%     
==========================================
  Files         167      167              
  Lines        8177     8179       +2     
==========================================
+ Hits         8054     8056       +2     
  Misses        123      123

Impacted Files	Coverage Δ
pybamm/expression_tree/state_vector.py	100% <100%> (ø)	⬆️

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[rtimms]

looks good to me thanks @tinosulzer