Side-effects of Changing the Unit attribute

If an AxisAxis has an active Unit attribute, changing its value (either by setting a new value or by clearing it so that the default value is re-instated) may cause the Label and Symbol attributes to be changed accordingly. For instance, if an Axis has Unit, Label and Symbol of ``Hz'', ``Frequency'' and ``nu'', then changing its Unit attribute to ``log(Hz)'' will cause AST to change its Label and Symbol to ``log(Frequency)'' and ``Log(nu)''. These changes are only made if the Unit attribute is active, and a MappingMapping can be found from the old units to the new units. On the other hand, changing the Unit from ``Hz'' to ``MHz'' would not cause any change to the Label or Symbol attributes.