Added `ZE`. Added `bwselect` with 4 inputs (`5ZI`). `Defined `&` for `XM`. Corrections

Added ZE: ifftn and ifft

Added bwselect with 4 inputs (5ZI).

Defined & for XM: second output. As suggested by @Suever

Small correction in Y@'s help

Solved issue 11. Thanks to @digital-carver for noticing!