Fix the _SIP1D class by making it subclass PolynomialBase and bringing it closer in functionality to other PolynomialBase subclasses.  Really it would be best if this subclassed Polynomial2D, but that will require a little refactoring of how the polynomial models work.  For one, it would be nice to easily customize the parameter name format.
Add a SIP forward transforation test. Correct a bug in calculating the number of SIP coefficients.
Split SIP/InverseSIP into separate implementations.  Fix inverse sip to
be a full polynomial, applied directly to the input.
add a test for inverse sip
Fix up what I guess is some stray code.
Normalize, somewhat, the format these header files are written in
Some slight variable renames for clarity
Fix this test for compatibility with the changes in #2360
Fix this test to work in light of #2360
Added changelog entry
Merge pull request #2177 from embray/fix-sip

Fix basic SIP model functionality