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
I wonder if this version of `parse_ra_dec` should be a public method.  The main issue is that it is currently written as a "success-oriented" function that cannot really fail for any input.  That is fine internally because within `SkyCoord` if the input isn't a coordinate then an exception will get raised later.  But for a public method named `parse_ra_dec` you expect that it will raise an exception if the input is not a valid RA-Dec string.

I was originally imagining that this method would be public (and would raise an exception for bad input), but that it wouldn't be explicitly called in SkyCoord.  This implementation is OK, but bear in mind that it will also require additional narrative documentation updates because it affects the public SkyCoord interface.

@eteq maybe you have an opinion?