Make lambda transform iterators copy-ssignable.
Make lambda transform iterators copy-ssignable.

eth-cscs/nestmc-proto,halfflat/nestmc-proto,halfflat/arbor,eth-cscs/nestmc-proto,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/arbor,halfflat/arbor,halfflat/nestmc-proto,halfflat/nestmc-proto,halfflat/arbor