Deprecate `post_install_step`, rename it to `post_processing_step`

[ocaisa]

When trying to create a post_install_hook for CUDA, I noticed that the easyblock already has a post_install_step that I didn't want to interfere with. The next opportunity to apply the hook was unclear and in fact is post_postproc_hook, which would be much clearer if the post_install_step had the more accurate name post_processing_step (I'm paraphrasing @boegel from Slack here, so he can correct me if I misrepresent the discussion)