path -- load path for attach and load

[was]

> > > In Mathematica, there's a $Path variable which has a list of directories
> > > to search when asked to load a file. Is there a corresponding thing in
> > > Sage?
> >
> > Sounds certainly like a good idea.
> 
> Thanks for considering it.

I have been meaning to implement this but haven't got around to it.
It has come up several times.   I think this would be very easy
to implement, actually -- literally 1 hour or so, plus adding
documentation and examples in the *right* places. 

[mpatel]

#378 is related.

[mpatel]

So is #5169.

[mpatel]

I'm closing this as a duplicate of #378.