convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

style: Filter out non-existing files

The style tool would fail if applying check against non-existing files.

trivial: style.py: Return error code

style: split filter logic into separate file

Soon we'll want to use this from shell scripts to exclude files from any
checks run by them.

Style: run style.py on python scripts

misc: add style-py.sh and use in style.py

This auto formats python code to pep-8 standards.

misc: increase functionality of style.sh

- split style.sh into style-c.sh and style-cmake.sh
- add style.py, which passes files based on extension to the correct
style tools and ignores files specified in a .stylefilter file (only at
the top level)
- style.sh now calls style.py and expects a list of files
- add style-changed.sh, which styles all changed files in a git repo.
- add style-all.sh, which styles all files in directories passed to it