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

See merge request !223

additional clang tidy checks

See merge request !226

Resolve "update clang-tidy to version 8"

Closes #86

See merge request !186

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

Updates for ExprTk

See merge request !224

[API] Resolve "move sqlite from postprocessing"

Closes #97

See merge request !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 !221

Fluctuating MRT

Closes #80

See merge request !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 !219