• fixed bug in format that resulted in several format codes ignoring width
• follow Python convention of right-justifying numbers by default.
• add Quantity.add() (adds a number to a quantity returning a new quantity)
• added # alternate form of string formatting.
• Change show_si to form (argument on Quantity.set_prefs() and Quantity.render()
• Added concept of equivalent units for unit conversion to documentation.
• Enhance UnitConversion so that it supports nonlinear conversions.