Integrating TimML and TTim

[dbrakenhoff]

Checklist:

• Add TimML and TTim to timflow/steady and timflow/transient preserving git history.
• Add documentation layer for directing users to steady or transient section, copy docs to new docs/steady and docs/transient sections.
• Add tests, also in tests/steady and tests/transient and keep as two workflows for now
• Setup readthedocs for the new repository
• Transfer discussions to new repo/org
• Duplicate open issues from both to new repo (issue numbering/referencing and link to author might/will probably break, but that's okay).
• Update Readme
• Consider crediting all previous contributors to TimML and TTim in this new repository somehow.
• ...