Use a neutral namespace for utility functions in the C++ course

[bernhardmgruber]

The C++ course extensively uses utility functions from the ach:: namespace, where ach refers to the accelerated computing hub (ACH). This is troublesome when the course material is used internally at NVIDIA in a context different from the ACH. We should use a neutral namespace in the notebooks and slides that does not need to be changed when the course material is used in different contexts.

Suggestions:

• help, helper
• util, utl
• aux

[brycelelbach]

I think it'll just be fine to use the ach name for everywhere. There's no need to rebrand it for DLI content.