avoidable multiprocessing guards on Windows and MacOS.

[dkriegner]

The powdermodel example script is somewhat complicated due to the fact that a main function needs to be defined for it to work on windows and MacOS.

xrayutilities/examples/simpack_powdermodel.py

Lines 28 to 36 in 4ff4dc8

	def main():
	"""
	dummy main function to enable multiprocessing on Windows and macOS.
	This is required to avoid starting of the multiprocessing code upon import
	of the script and is connected with the used multiprocessing start method.
	See also:
	https://docs.python.org/3/library/multiprocessing.html#the-spawn-and-forkserver-start-methods
	"""

There could be a workaround which would not require this for any platform and would make using PowderModel for users of these platforms much easier.
https://stackoverflow.com/questions/79397519/a-solution-to-multiprocessing-in-python-without-the-if-name-main-gu

Its however currently unclear to me how/if this is directly applicable for the multiprocessing code in powder.py.