Doc about the philosophy behind the unit system algorithm.

The algorithm which will be implemented is somewhat tricky to
understand but it is the only one allowing efficient symbolic
computations with dimensions, units and dimensions. The discussion
is based on several papers from physics literature.
Define the Dimension class with its __new__ function.
Add properties to Dimension: str and dimensionless.
Improve error messages and improve docstrings/tests for Dimension.
Define operations between dimensions.
Define MKS dimensions.
Define dimension systems.
Define the vector space properties of dimension system.
Define methods to get data from dimension systems.

Define str and repr, can extend the system to a new one, find
dimension in the system, and check system consistency.
Add natural system and	some doc.
Fix dimension display and tests (wrong name for a dimension).
Check consistency of the dimension basis.

Also add a way to check the dimension of the system.
Write the module for unit prefixes.
Create the Unit class.
Define properties of Unit, such as abbrev, factor with prefixes, etc.
Define operations for units.
Define unit systems.
Define methods for UnitSystem.
Stub for physical constants.
New function: sort units, display unit in terms of the base units.