With quantity output, ensure non-quantity input is treated as dimensionless
Added developer documentation for optional dependencies
Fixed typo
Removed custom ImportError for most optional dependencies
Implement suggestion by @embray
Merge pull request #2571 from mhvk/quantity-output-from-non-quantity-input

With quantity output, ensure non-quantity input is treated as dimensionless
Merge pull request #2616 from larrybradley/opdeps

Importing optional dependencies
documentation updates in coordinates
Merge pull request #2586 from taldcroft/coordinates-preferred

Included some additional minor fixes by @eteq post-PR
Added missing mention of represent_as in coordinate docs
Bring back helpful error message when ``.data`` is done on a data-less frame
Make SkyCoord creation fail if data is not provided