Handle missing OpenVDS, add warnings

[adnejacobsen]

Handle importing OpenVDS on MacOS and issue warning. Using methods/properties from the Cube class that uses OpenVDS will still raise exceptions.

I had to remove the type hints for Cube properties openvds_handle and openvds_handle_async as this raises exceptions on MacOS and I didn't find a way to do conditional type hinting. Open to suggestions for alternative solutions to this.

[equinor-ruaj]

lgtm