Add `half::f16` and `half::bf16` support

[swfsql]

• Closes https://github.com/rust-ndarray/ndarray/issues/1550
• This is a minor implementation, given that is easy to add new float types to ndarray.
• In best-effort, this adds tests/benches whenever both f32 and f64 were tested/benchmarked.
• Adds a feature mention on Readme.