Fixing warning

[rolanddenis]

PR Description

Removing warnings in DGtal when compiling with Eigen.

Checklist

• [N/A] Unit-test of your feature with Catch.
• [N/A] Doxygen documentation of the code completed (classes, methods, types, members...)
• [N/A] Documentation module page added or updated.
• New entry in the ChangeLog.md added.
• No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).
• All continuous integration tests pass (Travis & appveyor)

[dcoeurjo]

Thanks a lot.
Merging