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]
Remove the setter, replace with copy=False and related tests
Fix for missing equivalencies
Fix another an -> a
Normalize some of the wording from "one" to "you"
Add example of quantity-setting in columns
Add another example of modifying in-place in the modifying section
Add a warning about non-float columns and quantity
Change to explicitly giving a quantity view as suggested by @mhvk