Skycoord to accept more formats

This is just a dump of the skycoord related commits removed from #2846. This should not be merged as is. First the right place/function need to be found for it as it's representation specific.  Added features:

* acceptance of space separated <6 value format
* support for JHHMMSS.ss+DDMMSS.s format
Factor this out of the loop with something like:
```
is_ra_dec_representation = ('ra' in frame.representation_component_names and
                            'dec' in frame.representation_component_names)
```