Fixed a bug related improperly passing through parse_symtab call when extended USR is called recursively.

Fixed problem related to case-sensitivity of locally defined variables
Fixed problem related to compilation with latest version of clang (version 5.0.0 trunk 303036)
Minor code cleanup

WALBERLA_CHECK_FLOAT_EQUAL_EPSILON
WALBERLA_CHECK_FLOAT_UNEQUAL_EPSILON
WALBERLA_ASSERT_FLOAT_EQUAL_EPSILON
WALBERLA_ASSERT_FLOAT_UNEQUAL_EPSILON

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