Refine Owl type signatures

[ryanrhymes]

Current type signatures deviate from owl's original design rationale, and blur the functional boundary of various modules. We need to refile the type signature to better reflect owl's architecture.

[jzstark]

Calvin has done an excellent job of separating the Mat and Linalg aliases from the Ndarray modules. I think perhaps one thing that could be improved is the positioning/naming of the owl_algodiff_primal_ops module.

The original design is that CGraph functor stack takes Ndarray as input and produces the CGraph_engine type, and the Algdiff functor can take either Ndarray or CGraph Engine as input. Now after the change, a CGraph functor takes owl_algodiff_primal_ops module as input , and the Algdiff functor can take either owl_algodiff_primal_ops or CGraph engine as input, whereas owl_algodiff_primal_ops is a wrapper that aggregates Ndarray/Mat/Linalg functions.

According to this structure, I'm thinking that owl_algodiff_primal_ops as a standalone module could be separated out from Algodiff and renamed, in both owl-core and owl-base.