Handle cpu count exception

There was an issue on macosx. mp.cpu_count() can raise a NotImplementedError. If this happens, a warning is raised, and peaks_from_model (..., parallel=False) is return instead.

http://nipy.bic.berkeley.edu/builders/dipy-py2.6-osx-10.5-intel/builds/245/steps/shell_5/logs/stdio