Improved Pi Theorem implementation.

The nullspace of the dimensionality matrix is now obtained without NumPy assistance.
This eliminates NumPy as a requirement to use the Pi Theorem. Moreover, the implemented
gaussian elimination uses fractions (not floats) by default which is more proper for
the type of matrices usually found in this application.
Added derived dimensions to the unit registry.
Added mathjax to sphinx configuration to show math expressions in the docs.