Issue 1126 upwind

[valentinsulzer]

Description

Add upwind and downwind operators for convection.

Fixes #1126

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

❗ No coverage uploaded for pull request base (develop@377fc45). Click here to learn what that means.
The diff coverage is 98.36%.

@@            Coverage Diff             @@
##             develop    #1134   +/-   ##
==========================================
  Coverage           ?   97.92%           
==========================================
  Files              ?      247           
  Lines              ?    13957           
  Branches           ?        0           
==========================================
  Hits               ?    13668           
  Misses             ?      289           
  Partials           ?        0           

Impacted Files	Coverage Δ
pybamm/expression_tree/binary_operators.py	95.27% <75.00%> (ø)
pybamm/discretisations/discretisation.py	99.78% <100.00%> (ø)
pybamm/expression_tree/unary_operators.py	100.00% <100.00%> (ø)
pybamm/plotting/quick_plot.py	100.00% <100.00%> (ø)
pybamm/spatial_methods/finite_volume.py	97.95% <100.00%> (ø)
pybamm/install_odes.py	0.00% <0.00%> (ø)
pybamm/parameters/parameter_values.py	99.29% <0.00%> (ø)
...graphite_UMBL_Mohtat2020/graphite_ocp_PeymanMPM.py	100.00% <0.00%> (ø)
pybamm/solvers/casadi_solver.py	100.00% <0.00%> (ø)
...ectrolyte_exchange_current_density_Dualfoil1998.py	100.00% <0.00%> (ø)
... and 242 more

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[rtimms]

thanks @tinosulzer !