Changed the Heliographic frames constructor again.
Removed the default hlon and hlat values.
Now, either the frame shall be empty (no attributes at all), or it shall have attributes and consequently, values.
If it has attributes, and it is found that only hlon and hlat have been specified, then a default value for rad is added by the constructor. This makes Heliographic a true 3D frame.
Until Astropy roll out a default values mechanism for frame attributes which are part of the representation (i.e., not part of its FrameAttribute instances), such as 'rad', this shall be the means followed to prevent errors.