Add margins in mesh2vol

[kerautret]

PR Description

Add option to improve boundary extraction without hole:

New by using :
./converters/mesh2vol -i ~/mesh3D/al.off -o al.vol -r 128 -m 1
you will extract the perfect surfel boundary using volBoundary2obj:

Checklist

• Doxygen documentation of the code completed (classes, methods, types, members...).
• Main tool doxygen documentation (following existing documentation of DGtalTools documentation.
• Check if it follows the tools structure described in CONTRIBUTING.md
• New entry in the ChangeLog.md added.
• Update the readme with potentially a screenshot of the tools if it applies.
• No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).

[dcoeurjo]

agree, thanks