[Discuss] Move processor base classes from ezmsg-sigproc to ezmsg core

[cboulay]

Should we?
The main problem I'd like to solve is reusing the base classes in other ezmsg repositories (e.g., sources and sinks) without pulling in all of the dependencies that ezmsg-sigproc has.

Alternatively, we could make an ezmsg-axisarray repo given that all of the base classes assume AxisArray.
The protocols can be considered independently of the base classes.

Considerations:

• ezmsg core has an older Python requirement than sigproc (3.10.15)

[cboulay]

ezmsg core Python dependency has been bumped to 3.10.15 so this is no longer a concern for this issue.

So really we just need to decide if we want to pollute ezmsg.core or ezmsg.util with these extra classes.
An alternative could be adding a ezmgs.axisarray namespace module in this repo and migrating all the axis array-related code to this new module. To keep backwards compatibility, we would replace the old axisarray code locations with empty modules that simply import from the new locations.