Broadcast array arg in binary ops if it's a valid leaf array type (#51)

* broadcast arithmetic if arg2 is actx's array

* define and implement get_array_types

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

* adds test_leaf_array_type_broadcasting

* avoid using iff in the docs

replaced with only if

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

* adds docs for test, tests with scalars

* ArrayContext.get_array_types() -> ArrayContext.array_types

* bcast_actx_array_type -> _bcast_actx_array_type

Co-authored-by: Andreas Kloeckner <andreask@illinois.edu>

* formatting: remove unused import

* leaf array bcast types: better code placement

* make array_types a property

* docs: grammar

* better var naming: ary_types -> bcast_actx_ary_types

* Revert "make array_types a property"

This reverts commit e6b8b1b1860aadd6cd95269ca18dc22303b5b8b5.

* ArrayContext: make array_types a class attribute

Co-authored-by: Alex Fikl <alexfikl@gmail.com>
Co-authored-by: Andreas Kloeckner <andreask@illinois.edu>
Co-authored-by: Andreas Kloeckner <inform@tiker.net>