Separate production and testing versions

[ZacKeskin]

Currently PyCausality requires a number of dependencies (e.g. Nose, SciPy) which are in fact only used in testing. It is not uncommon to provide only the functional code when building for PyPi deployment, so consider removing testing-related code if this is not required for core functionality. The problem becomes clear when using PyCausality in non-local environments such as AWS Lambda or mobile, where environment management and storage issues become more difficult.

(Similarly, Matplotlib is required only for one method, so consider separating this out.)