Prepare compilation for C++17: std::any does not have `empty`

Stephan Seitzrequested to merge
seitz/walberla:c++17-any into master

std::any does not have a member empty, only boost::any. With this change waLBerla can also be compiled as C++17. We could also provide a generic function that works with both but this will not impede users to use the one version or the others. boost::any shouldn't be required anymore after a switch to C++17.

Edited by Stephan Seitz

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