PolynomialAxis: remove coefficient count limit

Firstly, it may be desirable to first create the axis before
programmatically adding a variable number of coefficients which are not
not known in advance.

Secondly, technically a polynomial axis can have less than three
coefficients. I think this check was just in place because LinearAxis
serves the same purpose if there are only one or two coefficients.
AbstractCalibratedAxis: override equals & hashCode

This allows CalibratedAxis instances to be compared for equality
in a non-trivial way. Equality is defined by the types, units and
particular equations of the axes all being equal.
Test each axis implementation's copy() method
DefaultLinearAxisTest: refactor field to local var
IdentityAxisTest: use slightly better test name