Went a step ahead and clarified that such layout-ordering assumptions
would be taken throughout PytatoArrayContext.

* mimic numpy's norm when ord==None

- When ord==None, numpy ravels the array and then computes the euclidean
  norm.

* test norm when ord is None

* fixup! mimic numpy's norm when ord==None

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

* implement PyOpenCLArrayContext.ravel

* fixup! implement PyOpenCLArrayContext.ravel

- order="K" was only correct for >0 stride values, instead renamed it to
  order="A" as per numpy.

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

Explains why pytato was being added as a requirement for downstream
packages.

- add pytato to requirements.txt of downstream projects to make them
  compatible with PytatoArrayContext

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