Require python 3 in cmake

The script to generate the particle properties uses python 3. Therefore, it should be listed as a requirement in CMakeList.txt

Yes, this is true. I believe the number of python scripts will increase a lot more in the future. This will really be a dependence for us.

created merge request !77 (merged) to address this issue

mentioned in merge request !77 (merged)

mentioned in commit dd44e5b7

closed via merge request !77 (merged)

mentioned in issue #152