Provide support for nonlinear calibrations

The calibration code in the img-metadata package no longer defines
calibration(int d) and setCalibration(double v, int d). There is no
assumption in the axis definitions that calibration is linear.

DefaultCalibratedAxis has gone away. LinearAxis is now the usual default.

There are places in the code that make linear assumptions. They are marked
with TODOs.
Calibration code sets origin point too
Improve some error messages
Remove obsolete comments
Add a comment
Avoid invalid test case
Document LinearAxis
Document UncalibratedAxis
Update javadoc
Update javadoc
Move axis implementations into own subpackage
Clean up linear space declarations
Add an axis backed by a general polynomial
Change contract of rawValue()

Some axis types cannot support calibrated value to raw value calcs. Think of
y = x^2. If given a calibrated y value there are two associated raw x
values. We cannot choose between them. In such cases we now return NaN as
the raw value.
Make update() aware of PolynomialAxis
Add tests for PolynomialAxis
Add a LogLinearAxis class with tests
Add a few more tests of LogLinearAxis
Flesh out UncalibratedAxis more fully
Define a general ExponentialAxis