Parallel: added AggregateMax functions

Added MPI_Allreduce wrapper with MPI_MAX operation.
Examples: added GridGen and FVDiscr examples

Added GridGen – simple parallel mesh generator
Added FVDiscr – simple two-point FVM scheme
Various small fixes

Fix few compile warnings and a small bug in Element::Mean.
Examples: clean up code

Added code comments, small fixes and improvements.
Documentation: updated README files

Added step-by-step install and examples guides