BUG: Panel.fillna with method='ffill' ignores the axis parameter (GH8251)
Adds support for method-based fillna along the specified axis of a Panel. Also adds some additional validation when calling fillna on DataFrame.

hah not sure we has this kind of collision before!

@staple thanks!

welcome to work on other issues of course!
 @jreback, thanks. I'll take another look at the "Good as first PR" tickets.
 Hah... sorry about that. I'm incorporating the new tests you wrote for sure. Thanks!
