Make ACTS code gcc-6.2 compatible (was ACTS-247)

Original author Christian Gumpert @cgumpert

All major experiments plan to upgrade to newer compiler versions and we should make sure that our code compiles without warnings using gcc-6.2. This gives us also the possibility to benefit from new language features (e.g. concepts).

Edited Mar 27, 2019 by Moritz Kiehn

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