Fixed bcs for the particle models

[brosaplanella]

Description

Changed how D is evaluated at the boundary condition to ensure that mass is conserved. I also changed some of the standard_output_tests to catch when mass is not conserved (before it didn't) and fixed some others.

Fixes #1420

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 #1421 (38a22fd) into develop (e1c51a2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1421   +/-   ##
========================================
  Coverage    98.27%   98.27%           
========================================
  Files          278      279    +1     
  Lines        16109    16131   +22     
========================================
+ Hits         15831    15853   +22     
  Misses         278      278           

Impacted Files	Coverage Δ
...ubmodels/interface/lithium_plating/base_plating.py	100.00% <ø> (ø)
...odels/submodels/particle/fickian_many_particles.py	100.00% <100.00%> (ø)
...dels/submodels/particle/fickian_single_particle.py	100.00% <100.00%> (ø)
...m/models/full_battery_models/base_battery_model.py	99.67% <0.00%> (-0.02%)	⬇️
pybamm/settings.py	100.00% <0.00%> (ø)
pybamm/plotting/plot.py	100.00% <0.00%> (ø)
pybamm/plotting/plot2D.py	100.00% <0.00%> (ø)
pybamm/plotting/quick_plot.py	100.00% <0.00%> (ø)
pybamm/plotting/plot_voltage_components.py	100.00% <0.00%> (ø)
pybamm/__init__.py	94.95% <0.00%> (+0.17%)	⬆️

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[valentinsulzer]

Thanks @brosaplanella !