Skip tests that require scipy if it's not available
Merge pull request #2647 from mhvk/tablecolumns-initialisation

Problem with TableColumns.__init__ for tuples, lists
Added some docstings to representation properties
Add Column headers for reading multiple apperture daophot output
Make frame class attributes read-only
update sgr-example
some code cleanup/clarification
move representation stuff lower down inside BaseCoordinateFrame
changed representation_names to representation_component_names and representatiounits to representation_component_units
change frame_specific_representation_info to use RepresentationMapping
Complete representation subclassing guide
remove representation `default_names` and `default_units` in favor of explict
A bit more cleanup and removal of unnecessary bits in representations
fixed components property to work on py 3.x
Docstring tweak in convert_input as discussed with @taldcroft in #2626
doc updates post-rebase
update sgr-example.py for new frame stuff
added recommended_units for representations and used those for builtin frames
Update tests for recommended units
Remove NoFrame and replace with defaulting to ICRS