Better python integration with klayout

[thomaslima]

This PR is designed to address #660 . So far, I have done the following changes to the macbuild installation:

• shipping a python binary alongside klayout, which imports the same libraries that the python interpreter within klayout does
• within that instance, I replaced sys.executable with that python binary rather than the klayout binary.

The motivation for this is to allow packages to install python packages with pip install by using a thread-safe subprocess call, e.g.:

subprocess.check_call([sys.executable, '-m', 'pip', 'install', 'numpy'])

With this PR, in the "heavyweight" HW distribution, this command places numpy into the klayout.app. It also does not crash klayout's python interpreter in case this fails.