problem poisson dd np: separated 'write' method from 'run' method

due to the recent viennafvm changes, we can now hold the problem description
in the state of the viennamini::problem, allowing a separation of the
simualtion from the IO
added 'laplace' problem and corresponding nin2d laplace example
removed old files
device/simulator: a segment can now be setup 'manually'

.. that is, no material name is given, only the raw material properties.
this is important to provide synthetic/academic properties for a 'test material'.
coupled viennamath::functionsymbols with viennafvm::quantities
introduced segment/material-dependent carrier mobilities
working on physics: want to implement material and temperature dependent mobility