add a CMakeCustom.cmake file, which is automatically used by ctest to handle custom...

add a CMakeCustom.cmake file, which is automatically used by ctest to handle custom commands. these were inside CtestPipol.cmake, and were ignored (see kitware.com/blog/home/post/27 for details about why they force this file). The coverage has dropped in the cdash, due to the fact that we now calculate the coverage based on simgrid files only, not on test files ..