Migrate tetgen formula to homebrew/science.

[goxberry]

TetGen is a mesh generator generally used for solving partial differential equations. It seems like a formula for tetgen would be better suited to https://github.com/Homebrew/homebrew-science. I am submitting a pull request shortly to homebrew/science that will update the TetGen formula; if I should submit it here instead, let me know.

[dpo]

I support this. Nothing in Homebrew currently depends on tetgen, so this wouldn't disrupt anything.

[dpo]

Thanks!