Let Quantity get its units from the `unit` attribute even for a non-Quantity
Try to convert to unit instead of isinstance check
Rescale when a non-Quantity input to Quantity has 'unit', instead of ignoring
Updated docstring to reflect additional options of input for Quantity
Add changelog entry

[ci skip]
Fix typo pointed out by @astrofrog
Add test for end-to-end behavior of going from a table to a coordinate

closes #1762
Merge pull request #2486 from eteq/quantity-init-table-units

Quantity initializer on a table doesn't know about units