#668 add evaluate function

[valentinsulzer]

Description

Add function to evaluate parameters more easily.
Fixes #668

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

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

[codecov]

Codecov Report

Merging #669 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #669      +/-   ##
==========================================
+ Coverage   98.54%   98.54%   +<.01%     
==========================================
  Files         169      169              
  Lines        8436     8441       +5     
==========================================
+ Hits         8313     8318       +5     
  Misses        123      123

Impacted Files	Coverage Δ
...ollector/effective_resistance_current_collector.py	96.29% <100%> (ø)	⬆️
pybamm/parameters/parameter_values.py	99.48% <100%> (+0.01%)	⬆️

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[rtimms]

looks good to me, thanks @tinosulzer !