Import entire tree of inheritance in astropy.io.ascii

In the current setup, not every class in io.ascii is documented, but only
those that are part of the top-level namespace io.ascii as defined in
io.ascii.__init__

For every class in there, sphinx sets links to the parent class. Thus, for
every class in this list, all parent need to be in this list, too.
This commit adds BasicData and BasicHeader, which are parents of
FixedWidthHeader and FixedWidthData, respectively.

Implement changes from PR discussion, mostly in ascii table reader docs