(towards #1995) Update source file headers

[sergisiso]

• Updated header of python, fortran and rst files to use SPDX and refer to root directory license
• Consolidated authors of previous headers and git log into AUTHORS.md

I haven't done other file types yet, e.g. shell, Makefiles, md. But GH is already struggling with the size of this PR, so it will come in a separate one.

[sergisiso]

@arporter Ready for review

Sorry for the length of this very repetitive PR. The following do not have the main header different_header.txt (but even this are repetitive)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d49fe29) to head (f5a1e6e).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #3508   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          397       397           
  Lines        55579     55579           
=========================================
  Hits         55579     55579           

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[arporter]

GitHub have clearly done some work to improve their handling of large PRs - this wasn't too bad. It's mostly inconsistencies in the dates but I see there are quite a few brand-new __init__.py files that I don't think are needed?

[arporter]

Also, since we're touching the surrounding comments, we should probably update the link to the SRS and replace it with github. We can also remove the (dead) link to the wiki page that used to be on Puma.

[sergisiso]

@arporter Addressed the comments and also removed the double-spacing the some copyright texts

[sergisiso]

@arporter This is ready for another review

[arporter]

Good stuff. Almost there now - just a couple of things slipped through.

[sergisiso]

@arporter Ready for another look

[arporter]

Thanks Sergi. All good now but I guess I should run the ITs to check.