Add VariableAxis base class to unify things
Extend VariableAxis as appropriate
Implement calibratedEquation() in VariableAxis

For axes whose variable values are tracked in the variables table of
VariableAxis, it is a simple matter to replace the variables'
occurrences in the general equation with the specific values given by
that table. This obviates the need to implement calibratedEquation() in
these subclasses.
VariableAxis: add missing javadoc
Rename Abstract*VariableAxis classes

Since they have no corresponding interfaces, we drop the "Abstract"
prefix. To keep the classes together lexicographically, this change opts
to use the naming pattern "Variable[1234]Axis" rather than keep the
numbers spelled out.