getMaximumPenetration()
getNumberOfContacts()
getNumberOfContactsTreated()

- Bug occurred if a BufferSystem (or communication scheme) was copied
  and the first object went out of scope.
- Internally a pointer was kept which was still pointing to the original
  BufferSystem

Fixes #16

- can now use the current time step of an ITimeloop object as reference
- can now export data to SQLite databases
- added option to refresh cell counters on each call

Added compile-time check for vector indexing when using constant values

Updated multi-sub expression operator to return final sub-expression type.
Updates to exprtk_disable_return_statement macro for disabling return statements and associated exceptions at the source code level.
Various other updates

If you have access to the old closed source waLBerla repository, you may restore the complete history with the following command
git fetch git@i10git.cs.fau.de:software/walberla.git 'refs/replace/*:refs/replace/*'