Possible crashes on OS X

[molpopgen]

Several of the functions of fwdpy11.TreeIterator return numpy arrays whose data are stored in a C++ vector. The method used for this is from this post.

The symptoms are only apparent on OS X and they take the form of semi-random crashes. They do not happen all the time, nor do they happen in the same part of the code.

Changing the implementation to return a copy of the data seems to eliminate the issue altogether, but this is not a desirable solution.

Several things remain unclear:

1. Is this a symptom of compiling fwdpy11/dev using the Xcode clang, yet the Python used is from conda, which is presumably compiled with clang_osx-64? This would be an ABI issue and not an actual bug.
2. Or is this a "real" bug. So far, it cannot be reproduced on Ubuntu.

Sadly, ruling out 1 is complicated by it being quite difficult to get clang_osx-64 working properly on my system. Another option is to install Python3 by another means + all dependencies, and see what happens.

cc @apragsdale