Implement CTF-IR event-fields getters

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Docs: CTF-IR sequence's length field must be unsigned

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Tests: Test CTF-IR event-field's getters

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Update CTF-Writer and CTF-IR copyrights

Changed the copyrights to stay remain in line with our usual
politic of authors retaining their copyright on the code they
contribute.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add Python bindings for CTF-IR event-field getters

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>