[Output Saver] Initial extension of outputsaver for SVG

[oloapinivad]

PR description:

This will implement the support for SVG figure output.
I started very simply, noticed a couple of issues

• the add_pdf_metadata and add_png_metadata are still part of AQUA core, and should be transferred here I guess
• we are using a bit clumsy structure for saving, not based on the extension but on the save_${extension} flag. I would possibly refactor it
• I think we can mostly refactor the structure so that it supports all the format available in matplotlib: we just need a way to distinguish between raster and vectorial images.

Issues closed by this pull request:

Close #135

People involved:

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