additional clang tidy checks

See merge request walberla/walberla!226

Resolve "update clang-tidy to version 8"

Closes #86

See merge request walberla/walberla!186

[ADD] boost*
[ADD] mpi*
[ADD] openmp*
[ADD] portability

Updates for ExprTk

See merge request walberla/walberla!224

[API] Resolve "move sqlite from postprocessing"

Closes #97

See merge request walberla/walberla!222

To break unneeded dependencies in postprocessing the SQLite part is
moved to its own module and namespace.
Therefore in the include and the function calls
"postprocessing" hast to be removed or replaced by "sqlite" e.g.:
#include "postprocessing/sqlite/SQLite.h" ->
#include "sqlite/SQLite.h"

postprocessing::store... -> sqlite::store...

Specialize std::hash<T> for Cell, Vector2, Vector3

See merge request walberla/walberla!221

Fluctuating MRT

Closes #80

See merge request walberla/walberla!220

This was lost in the migration from boost to std as boost::hash<T> (and thus boost::unordered_map<T,V>) uses hash_value<T> if available and std::hash<T> (and thus std::unordered_map<T,V>) does not.

Add cmake option WALBERLA_LOG_SKIPPED

See merge request walberla/walberla!219