Fixing ArithmeticalDSSComputer aliases

[rolanddenis]

PR Description

This PR modifies two things:

• defining StandardDSS4Computer and NaiveDSS8Computer as templated aliases of ArithmeticalDSSComputer instead of derived classes, thus fixing issue Static assertion raised in SegmentFactory when using SaturatedSegmentation #1483
• fixing NaiveDSS8Computer adjacency that was previously set to 4 instead of 8 (unless I misunderstood something...)

Checklist

• 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)

[dcoeurjo]

Thanks @rolanddenis

[dcoeurjo]

Perfect, thx