Edge buffer is lazily initialized

[molpopgen]

When we start a sim, we are not allocating a list size equal to the number of currently-alive (founder) nodes. Things tend to work because the common use case is to evolve a lot so the necessary allocations happen. But, for cases like #624, when we re-enter the evolve code path, we can fail to have the correct number of lists allocated when we simplify every generation, including the first. The result from the Python side is IndexError, which ultimately originates from a call to fwdpp::nested_forward_lists::head.