Coherent policy needed on array types

[bwohlberg]

This issue is related to both typing and explicit array conversion and creation (the latter form being closely related to the subject of #93). At the moment we have a somewhat random mix of Array and JaxArray type specifications within our code. A coherent policy is needed as a guide to addressing the existing random usage, and ensuring that it doesn't persist into new code.

The simplest policy is obviously to simply require that all scico functions and methods take and return JaxArray types. Is this both feasible and desirable? If not, what are the counter-examples?