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
Better to not use try / except here because that might end up hiding an unexpected exception.  Something like:
```
match = JPMRE.match(coord_str)
if match:
    coord = match.groups()
    ...
else:
    coord = coord1
```