moving OpenFST and OpenGRM to homebrew-science

As per the discussion in
https://github.com/Homebrew/homebrew/pull/31888, the maintainers feel
that homebrew-science is the right place for OpenFST and OpenGRM. I have
moved two homebrew formulae here (openfst and opengrm-ngram) as well as
a new one, opengrm-thrax, which also depends on OpenFST.

OpenGRM-thrax is a tool for compiling morphological analyzers and
context-sensitive rewrite rules into OpenFST automata.