Issue 765 compute grad fem

[rtimms]

Description

Added the gradient operation in the finite element method. This has then been used to compute the ohmic heating in the current collector

Fixes #765

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. If necessary, also add to the list of breaking changes.

• 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 #767 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #767      +/-   ##
==========================================
+ Coverage   98.36%   98.38%   +0.01%     
==========================================
  Files         179      179              
  Lines        9871     9894      +23     
==========================================
+ Hits         9710     9734      +24     
+ Misses        161      160       -1

Impacted Files	Coverage Δ
pybamm/spatial_methods/spatial_method.py	95.12% <ø> (ø)	⬆️
...rmal/xyz_lumped/xyz_lumped_2D_current_collector.py	100% <ø> (ø)	⬆️
...thermal/x_lumped/x_lumped_2D_current_collectors.py	100% <ø> (ø)	⬆️
pybamm/expression_tree/unary_operators.py	96.18% <ø> (ø)	⬆️
pybamm/spatial_methods/scikit_finite_element.py	98.23% <100%> (+0.27%)	⬆️
pybamm/expression_tree/symbol.py	98.29% <0%> (+0.42%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5b27183...dc2543f. Read the comment docs.

[valentinsulzer]

Looks good to me!

[valentinsulzer]

np.testing.assert_array_less(0, ans) would give more informative failure messages