Issue 785 init concs

[valentinsulzer]

Description

Allow initial conditions in the particle to depend on x

Fixes #785

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 #786 into master will increase coverage by 0.01%.
The diff coverage is 99.33%.

@@            Coverage Diff             @@
##           master     #786      +/-   ##
==========================================
+ Coverage   98.44%   98.46%   +0.01%     
==========================================
  Files         179      181       +2     
  Lines        9980    10160     +180     
==========================================
+ Hits         9825    10004     +179     
- Misses        155      156       +1

Impacted Files	Coverage Δ
pybamm/expression_tree/functions.py	100% <ø> (ø)	⬆️
pybamm/models/submodels/particle/base_particle.py	93.33% <ø> (ø)	⬆️
pybamm/expression_tree/input_parameter.py	100% <100%> (ø)	⬆️
pybamm/expression_tree/independent_variable.py	100% <100%> (ø)	⬆️
...odels/full_battery_models/lithium_ion/basic_dfn.py	100% <100%> (ø)
...odels/full_battery_models/lithium_ion/basic_spm.py	100% <100%> (ø)
pybamm/expression_tree/state_vector.py	100% <100%> (ø)	⬆️
...ollector/effective_resistance_current_collector.py	96.29% <100%> (ø)	⬆️
.../models/submodels/thermal/isothermal/isothermal.py	100% <100%> (ø)	⬆️
pybamm/meshes/meshes.py	98.9% <100%> (+0.02%)	⬆️
... and 18 more

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[rtimms]

looks good to me, thanks @tinosulzer