Extend unit test coverage for phynx-0.11 for python-phynx

The unit tests will be extended to cover the existing features in phynx. This needs to be done in conjunction with the documentation. It clarifies how the features are intended to be used by supplying working examples. nose will be used in conjunction with the coverage package.

Support for and propagation of a physical quantity's precision/error for python-quantities

In general, physical quantities have three important properties: a magnitude, dimensionality, and precision. quantities should be able to specify the precision or measurement error, and propagate such information appropriately during arithmetic operations.