new dune.generator.setModuleLog and envvar DUNE_LOGMODULES to speedup C++ development

This MR implements two options for improving the development of the C++ side of the Python binding. In both cases the jit module names are logged in a file scriptname.modules. If that file exists at startup of the script all modules listed in the file are rebuild if dependencies have changed. This can be done in parallel using make -jn.

This can be useful for C++ developers to speed up running scripts when changing the Dune header files.

1. Call dune.generator.setModuleLog( __file__ , procs=n) at top of a script - n processes will be used.
2. Set the environment variable DUNE_LOGMODULES to the number of processes to use for rebuilding. So if this variable is greater than zero logging will take place automatically and parallel rebuilding when rerunning a script multiple times.