Add an option to guess the start of data for reading CDS files
Add cds_malformed.dat test file
Add docs for setting data_start to 'guess'
Updated CHANGES.rst
Add comment line in test referencing issue numbers
Update equivalencies.rst

Removed (nearly) repeated text.  I kept the most recent additions.
Corrections to ensure Distance can be compared.  Closes #2206
Merge pull request #2241 from taldcroft/cds-malformed

Add an option to guess the start of data for reading malformed CDS files
Update construct_table.rst

Fixed typo.
fixed AiryDisk2D model
updated changelog
updated changelog
Updated AiryDisk2D model docs
Merge pull request #2252 from larrybradley/patch-2

Update construct_table.rst
defined module-level constant for AiryDisk2D model; fixed bug with peak-pixel amplitude
changed AiryDisk2DKernel input parameter from width to radius to match AiryDisk2D model
Updated test for AiryDisk2D model for non-unity amplitude
Added reference link for AiryDisk2D model
defined AiryDisk2D constant in the model class
Update unified.rst

Fixed typos.