Store conversion data in class attributes of UnitConvert

[goerz]

The _read_units_definition method should be a class method, and all the attributes it sets class attributes.

Various classes such as UnitFloat and Pulse keep an instance of UnitConvert for internal unit conversion. In the case that a user adds new units, these should be available to all converter instances. It might also make sense to disable the possibility of having alternative internal units than au.