Switch repository licensing to MIT and add MIT headers to maintained Fortran sources

[Copilot]

This updates the project from GPL to MIT at both repository and source-file level. It standardizes license text across maintained Fortran sources while explicitly preserving third-party fftpack files.

• License file update

  • Replaced root LICENSE contents with the standard MIT License text.
  • Included project copyright notice:
    • Copyright (c) 2023-2026 Jason Christopherson

• Fortran source header standardization

  • Added a full MIT banner comment block to all maintained .f90 files in:
    • src/ (excluding src/fftpack/**)
    • examples/
    • test/
  • Kept all executable code unchanged; only prepended license/copyright comments.

• Explicit exclusions

  • No changes to src/fftpack/** (non-owner-authored sources).
  • No changes to doc/src/** generated documentation copies.

! MIT License
!
! Copyright (c) 2023-2026 Jason Christopherson
!
! Permission is hereby granted, free of charge, to any person obtaining a copy
! of this software and associated documentation files (the "Software"), to deal
! in the Software without restriction...