Add a function that outputs matrices as .csv files.

[ckarampa]

NEW:

• Pretty much as the title says, a function that exports gsMatrix as .csv files has been added.
• Optionally column headers can be specified

IMPROVED:

• (my quality of life)
• inputOutput_example.cpp now also exports the geometry's coefficients to a .csv file, as an example.

FIXED: ...
API:...

Please consider the following checklist before issuing a pull request:

• Have you added an explanation of what your changes do and why
  you'd like us to include them?
• Have you documented any new codes using Doxygen comments?
• Have you written new tests or examples for your changes?

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