[Ifeffit] odd feff6 behavior

[Matt Newville]

Hi Bruce, 

I agree 100% with your sentiments.  The problem you had was with
Feff's design (especially that it stopped when it could have
gone on).  Feff is not particularly amenable to running as a
"module" or embedded in another program.  Even as a standalone
executable run in "batch mode", Feff isn't very good at at
giving meaningful error messages to the user.

Fortunately, we can change the source code for Feff6, even if it
is not necessarily easy to do so.  I've made several changes to
Feff6 already, and am willing to make more.  So far, no one else
has volunteered to take this on.

As a relevent example, there are currently more than 50 places
where feff6 may end execution because of an error (fortran
'stop').  Right now, Ifeffit only does this on reading a
corrupted PAD-format data file, which aren't used with
artemis/athena. Ideally, all of these could be replaced with
"write meaningful message, and go on as best as you can".  I'm
not sure how easy that will be.

--Matt