Fixed handling of the boundary conditions for explicit solver - introduced a method Problem::applyBoundaryConditions.

Only MPICommunicator and DistributedGrid is implemented.

Allow existance of non setup synchronizer object. MeshFunction creates grid synchronizer automagicli with binding.
(only if distr grid was chnged)

There is shor test of this stuff for 1D 2D and 3D.

Relpaced tabs with 4spaces.

Adding Gnuplot writer for vector field.

I think this is much better because even operators inherit from Domain
and operator.getDimensions() is very confusing.