Issue 1194 initial conditions

[valentinsulzer]

Description

Add a function to initialize a model with a different model, fixes #1194
Reformat model.new_copy so that any changes to the model are kept, fixes #1205

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 #1278 (2012162) into develop (9d42dfe) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           develop    #1278    +/-   ##
=========================================
  Coverage    98.07%   98.08%            
=========================================
  Files          270      270            
  Lines        15030    15152   +122     
=========================================
+ Hits         14741    14862   +121     
- Misses         289      290     +1     

Impacted Files	Coverage Δ
pybamm/expression_tree/operations/simplify.py	93.35% <ø> (ø)
...ybamm/expression_tree/operations/unpack_symbols.py	100.00% <ø> (ø)
pybamm/meshes/meshes.py	97.75% <ø> (ø)
...bamm/models/submodels/external_circuit/__init__.py	100.00% <ø> (ø)
pybamm/discretisations/discretisation.py	99.78% <100.00%> (+<0.01%)	⬆️
...bamm/expression_tree/operations/replace_symbols.py	100.00% <100.00%> (ø)
pybamm/models/base_model.py	99.08% <100.00%> (+0.07%)	⬆️
...m/models/full_battery_models/base_battery_model.py	99.67% <100.00%> (-0.02%)	⬇️
...models/full_battery_models/lead_acid/basic_full.py	100.00% <100.00%> (ø)
...odels/full_battery_models/lithium_ion/basic_dfn.py	100.00% <100.00%> (ø)
... and 7 more

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[brosaplanella]

Looks great Tino, thanks!