Add alternate outer product (#46)

* Make broadcast settings visible on users of with_container_arithmetic

* Remove stray debug print

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

* Fix _outer_bcast_types attribute name

* add alternate outer product

* add empty line

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* add comment explaining _outer_bcast_types

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* add more detail to docstring first line

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* rename is_scalar -> treat_as_scalar

* treat non-object numpy arrays as scalars in outer

* remove use of deprecated is_array_container

* clarify usage of isinstance(..., ndarray) for object array detection

Co-authored-by: Andreas Klöckner <inform@tiker.net>

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