add unified access for the implementation of DiscreteFunction

Improved test and range initialization of DiscreteFunction

extend the DiscreteFunctionTest to test for construction with std containers

Add range type to solution() and oldSolution() functions

Added init-finalize also for vectors.
Require gather/scatter operations instead of direct vector/matrix access.
Rewritten the interpolation.
Added ParallelIndexSet and DOFMapping to be used also in other backends.

cleanup oldSolution in ProblemInstat, so that it can be used in expressions and allows interpolation during grid changes

added virtual destructors, wrong extension string in mesh reader, forward declaration in operation::composer

cleanup of source-tree, new directory common for commonly used functionality, advanced addMatrixOperator syntax, OperatorTerm creation from Operators.