race condition in context.py

[eirrgang]

Since there is not a filter to only create a working directory on the rank that needs it, there is a race condition between checking for the existence of a directory and attempting to create it from Python (which does not have an equivalent of mkdir -p).

I can fix the race condition now, but this may indicate a need for an additional element of the Context API to perform infrastructure tasks redundantly or not on each worker depending on the Context. However, in the case of working directories and file paths, the need does not exist because of the way we will be managing data placement in a near future release. ref #96 and #79