Add inverse to QFT implementation

[vaferreiQMT]

Howdy, I hope your holidays and the start of the new year have been great!

While the refactor of the QFT implementation in #604 remains in the backlog, I think it would still prove useful to have an inverse QFT for completeness. Since I needed this for another project and it is a fairly minor change I figured it might be worth tidying it and submitting a PR.

Main changes:

• Inverse QFT: adapted from the forward implementation. Controlled via an inverse parameter, which is a breaking change, but I think it makes sense to keep the QFT and inverse together. Happy to split it if you disagree.
• Inverse QFT unit tests: using the existing tests for the QFT with an additional generator for the inverse parameter.
• QFT docs: documentation for the QFT methods. Maybe not to full fit and finish, but hopefully better than nothing.

Cheers!