[NewtonSolver] Make compile with g++5.3

• A known in bug in g++5.3 leads to a compiler error due to the trailing return type of the generic lambda in the supportsPartialReassembly method
• Replacing that method with a struct providing the ()operator should fix that issue

fixes #460 (closed)

@becker : Could you please check on your machine? You are probably the only one with g++5.3 :-)