Fix #2559: Make equivalencies work correctly with compose.
Add another compose-to-equivalencies example.
Fix tests
Fixed error in io.ascii on Mac
Added a --repeat option for testing
Added repeat option to test()
Correctly detect and handle invalid input world coordinates.
Add CHANGES entry
Add `to` method to Column
Add `to` method tests
Split ``to`` method into a ``quantity`` property and a ``to`` method

They are distinguished by ``to`` requiring a unit where previously it
could be just None
Add changelog entry
Add some documentation on Quantity <-> Table interactions
typo fix pointed out by @astrofrog
Fix for possible rounding problems in test mentioned by @mhvk
removed example that might lead users astray

Because table quantities won't behave right for some things, like
trig functions where the quantity conversion doesn't happen.
add equivalencies and fix up docs for to/quantity
Update docs to make it clearer that columns don't always work like quantities
Added setter for quantity attribte
Fixed some typos pointed out by @mhvk

[skip ci]