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
Keep things manageable for you and users by saying that `coord_str` is one string containing a single coordinate value.  That is effectively what your code is handling.  So start with `Parse RA and Dec values from a coordinate string`.