Fix derivatives and add tests
add explanation of dictionary keys
Merge pull request #1534 from nden/model-derivatives

Model derivatives - minor changes and tests
Have separate variables for setup_requires and install_requires.
Assigning a quantity to a table column sets the unit on the column correctly
Added an example to the documentation
Removed unnecessary import
Added CHANGES.rst entry
Added scalar quantity test
Fixed API links
Merge pull request #1561 from mdboom/setup/install-requires

Add separate variable for install_requires
Merge pull request #732 from astrofrog/table-set-units-from-quantity

Set column units when initializing Column or Table from a Quantity
Control whether metadata merge conflicts result in a warning, an error, or nothing (only in vstack for now)
Improved description for ``metadata_conflicts`` and add the option to ``hstack`` and ``join``.
Ensure that an exception is raised if metadata_conflict is not one of e expected values.
Fixed typo
Use custom warning class
Always pick the right key
Added tests for metadata merging with hstack, vstack, and join
Started working on also providing control over metadata conflict errors/warnings for column metadata