Permutation matrices research.

docs: Added TODO.md list.

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TODO.md
··· 1 + 1. [ ] `rg TODO src/` 2 + 2. [ ] Clean up git blob: 3 + `3fe94d749f62112dda1ff5f3fe68606b0237fe6f (c9e85c700dfd5ee8dcef9a6380731d75d59ee907:costas_10x10.txt)` 4 + You can do this with `git filter-branch` (https://www.deployhq.com/git/faqs/removing-large-files-from-git-history).