add conj() and arctan2() (#83)

* add conj()

* flake8

* add arctan2()

* bugfix arctan2 negative x

* add to doc

* quieten mypy

* generate IndexLambda for arctan2

* start generalize _apply_elem_wise_func

* generalize _apply_elem_wise_func

* Cultural Learnings for Make Benefit Glorious Nation of Mypy

* Fix generalization of _apply_elem_wise_func

* add tests

Co-authored-by: [6~ <inform@tiker.net>