Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

python2 --> python3

Update all scripts and build system to call python3, given python2's
upcoming doom. Use sys.maxsize instead of sys.maxint in one script
(maxint does not exist in python3).

style: run autopep8 on python files

manual: fix stray brace in output

Invocation headers had a stray brace which broke the markdown output

manual: remove gen_invocations.py unused functions

manual: group generated API methods by object type

This change generates doxygen groups for each object type, which allows us to create sections in
output documents for each object. This has the advantage that we can later label those sections and
link to them from the main document. Additionally, it improves nagivation of the API docs.

manual: fix script licenses

manual: Docstrings for gen_invocations script

manual: Remove unused import

manual: Less fragile handling of arch types

manual: Validate interface files against dtd

manual: Docstring for gen_invocations stript

manual: Build system compatible with python3

manual: Tool for generating minimal obj inv header

JIRA: SELFOUR-606