move mpi stubs into walberla::mpistubs namespace

Closes #18

See merge request !17

Replaces exception thrown on getBlock with NULL

See merge request !12

Adding special documentation for modules (pe)

See merge request !14

New PE Body: Union

See merge request !4

WALBERLA_CHECK_FLOAT_EQUAL_EPSILON
WALBERLA_CHECK_FLOAT_UNEQUAL_EPSILON
WALBERLA_ASSERT_FLOAT_EQUAL_EPSILON
WALBERLA_ASSERT_FLOAT_UNEQUAL_EPSILON

Change Box to Capsule in rigidbody communication header

See merge request !13

getMaximumPenetration()
getNumberOfContacts()
getNumberOfContactsTreated()

two versions available now via CMake switch:
   - by default the Python module is built without LBM module, since
     lbmpy can be used for this. Reduces compile time substantially.
   - if the LBM is needed in python, for example for the lbmpy testsuite
     it can be included by setting the CMake switch
     WALBERLA_BUILD_WITH_PYTHON_LBM

- 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

Merge branch '17-does-not-compile-error-cannot-convert-walberla-real_t-aka-double-to-real_t-aka-float' into 'master'

Resolve "Does not compile: error: cannot convert ‘walberla::real_t* {aka double*}’ to ‘real_t* {aka float*}’"

Closes #17

See merge request !9

Enhanced lbm::PerformanceLogger

See merge request !8

Various updates to ExprTk

See merge request !7

- 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

bugfixes and extensions for free_surface support

See merge request !3

Minor updates to ExprTk

See merge request !5

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