Integration of Hard Contact Solvers into MESA-PD

See merge request walberla/walberla!218

Fixed small bug in pe's HCSITS solver and added overrelaxation parameter

See merge request walberla/walberla!223

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