Issue 1203 save to matlab

[valentinsulzer]

Description

Add error message and option to use different name if variable name contains spaces or special characters, so that matlab can read it
Fixes #1203

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 #1206 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1206   +/-   ##
========================================
  Coverage    97.93%   97.93%           
========================================
  Files          247      247           
  Lines        14036    14048   +12     
========================================
+ Hits         13746    13758   +12     
  Misses         290      290           

Impacted Files	Coverage Δ
pybamm/simulation.py	96.81% <ø> (ø)
pybamm/solvers/solution.py	99.34% <100.00%> (+0.05%)	⬆️

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[brosaplanella]

Looks good, thanks Tino!