factor: Add/update copyright notices as necessary

[nbraud]

I discovered yesterday that I accidentally failed to update/preserve those when
moving code around (#1525), deleting code (#1529), or adding my own
contributions.

This rectifies it, also adding years (based on git history) for all copyright
lines; I took the liberty not to track the individual changes for which
copyright is claimed, since that information is in git and tacking it by hand
would get out-of-hand very quickly.

[sylvestre]

I am clearly not a fan for having a list of contributors in header (too strong sense of ownership, when do we add names, etc). we should have a global author file at the root directory.
but this is a discussion for another day!

[nbraud]

I am clearly not a fan for having a list of contributors in header (too strong sense of ownership, when do we add names, etc). we should have a global author file at the root directory.
but this is a discussion for another day!

Yes, this is not my preferred solution either, but having it partially-there was strictly worse (and changing this convention for the entire project felt waaay outside my remit.)