Fix bug in ImageLinearCellEmbedder

[JacquesOlivierLachaud]

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PR Description

This PR fixes a bug in class ImageLinearCellEmbedder when the image was made of unsigned values (for instance, a grayscale image made of unsigned char). In this case, the difference y2-y1 may sometimes be negative but the type forbids the value to be negative. The solution is to first cast values to double, then make the difference.

Checklist

• Unit-test of your feature with Catch.
• Doxygen documentation of the code completed (classes, methods, types, members...)
• 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)

[JacquesOlivierLachaud]

Seems to pass everything except too long to compile by travis.

[dcoeurjo]

Perfect. Thanks.