Return policy should be copy for Sim1.iterator

[james-d-mitchell]

In v0.4.1, we have:

[x for x in S.iterator()]

contains only the empty word graph.

libsemigroups_pybind11/src/sims1.cpp

Line 217 in 71be8d0

"iterator",

[james-d-mitchell]

Or maybe this isn't necessary, perhaps it's just desirable to provide (if it doesn't exist already) a convenient way to copy the action digraph pointed at by x.