PR Description

New option to change the alpha value of displayed meshes

Checklist

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