Convert the var dim assignment ckernels to the CRTP method
Add class arrmeta_holder for helping manage type+array metadata

This is for contexts where one doesn't want the overhead
of allocating a full-blown reference counted nd::array, e.g.
inside a ckernel.
Add stub documents for all the different ckernel types
Create ckernel_deferred for rolling window
Implementing rolling ckernel deferred
Get creation of generic rolling ckernel working