Add option for initial soc in build

[aabills]

• add option in build to initialize soc
• remove added dependency
• add test

Description

move initial soc setting to its own function, which can be called from build
Fixes #2372

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

[valentinsulzer]

Looks good to me

[codecov]

Codecov Report

Base: 99.66% // Head: 99.66% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (a859382) compared to base (7f06355).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2375   +/-   ##
========================================
  Coverage    99.66%   99.66%           
========================================
  Files          269      269           
  Lines        20201    20206    +5     
========================================
+ Hits         20134    20139    +5     
  Misses          67       67           

Impacted Files	Coverage Δ
pybamm/simulation.py	100.00% <100.00%> (ø)

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