Force a single-value per element for elementwise reductions (#195)

* Force a single-value per element for elementwise reductions when applicable

* Fix RST formatting in docstring

Co-authored-by: Kaushik Kulkarni <15399010+kaushikcfd@users.noreply.github.com>

* Fix surface area computation in dt helper function

* Simplify surface area branches

* Update comments in dt utils

* Branch surface area computation based on actx broadcasting properties

* Test geometric factors using lazy actx

Co-authored-by: Kaushik Kulkarni <15399010+kaushikcfd@users.noreply.github.com>
Co-authored-by: Andreas Klöckner <inform@tiker.net>