added contour selection routines for rectangles and circles.
Bumped version to 0.52
updated documentation
removed the printXX_sel commands, now the print XX commands use the
selected nodes in the mesh (if any), which is more flexible and
leads to less code and less commands.
 updated thin film example to the last changes (i.e. deselect before printmesh
updated CSiSTD example
updated thin film example
Several changes:
1: added export functions for current densities and electric fields
2: added possibility to define polygons within the PVMOS input file
3: in diode.c I now use 8.6173323478e-5 eV for k_B (used to be the less accurate 8.617e-5 eV)
updated documentation