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
If there are > 2 and < 6 components, but no + or - then this will raise an `IndexError` here that will be confusing.