Add convenience ctors to CalibratedAxis hierarchy
Normalize setter arguments

The usual pattern is for the setter argument to be named after
the setter itself, as well as the associated instance field.
Make variables final when feasible
Surround equations in {@code} blocks
Fix various minor formatting issues
Use convenience ctors where applicable
Make instance fields private instead of protected

For details, see:
http://developer.imagej.net/coding-style#Private_over_protected_fields_and_methods
PolynomialAxisTest: update test method names

They now reflect the renamed equation method names as of
59062e53d091c69ecdd9bd4d794fbc9eaef272f7.
WIP: generalize particularEquation() implementation