Python 3.2 introduces the `itertools.accumulate()`. Before, we were
using less flexible custom code for performing the same task, in
`networkx.utils.misc.cumulative_sum`. This commit replaces calls to
`cumulative_sum` with calls to `itertools.accumulate()`.

Since we currently still support Python 2.7, this commit adds a fallback
equivalent definition of accumulate that matches the Python 3.2
implementation. Once support for Python 2.7 is dropped, all of this code
can be removed, and calls to `networkx.utils.accumulate()` can be
replaced by calls to `itertools.accumulate()`.