fix gt::copy for const data type spans and containers

[bd4]

If the source has a const data type, no signature will match and compile will fail, e.g.

error: no instance of overloaded function "gt::copy" matches the argument list
            argument types are: (gt::gtensor_span<const real_t, 2UL, gt::space::thrust_host>, gt::gtensor<real_t, 2UL, gt::space::device>)`

there is no reason for this to not work.