Some slight tweaks to CompositeModels for the v0.3.2 release.  This doesn't add much improvement or fix any issues--instead it addresses issues like #1836 by being clearer about what these classes don't currently support.  It also slightly improves their text representations.
Make Row, Column, MaskedColumn, TableColumns be class attributes
Add initial tests of subclassing components
Make the user-subclassable attributes be public (no _)
Refactor pprint funcs into class to allow formatting customization
Make subclass testing more boring
Add subclassed Table into test fixture for broad testing
Add docs on subclassing
Update CHANGES.rst
Add top-level reference to subclassing, fix doc problems
Change t.pprint() to print(t) to fix doctest errs on Mac
Fix RST docs to run correctly and be more clear
Fixes for remote-data tests prior to APE3 work.
Added Redshift model for wavelength.
Added description to Redshift z parameter.
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.