[Ifeffit] formula checking

[Matt Newville]

Hi Bruce, Peter,

Ifeffit does print a warning message about a syntax error on using
'//'.  But it's completely believable and forgivable that Artemis
doesn't catch this.  

Bruce, would it help if a flag was set indicating 'syntax error'
in addition to printing the warning message?  That way you could
wrap each of the definitions with a perl function that did the
ifeffit command and check for syntax errors, doing something
sensible (popping up a warning dialog box, for example) on a
syntax error.  That might be better than checking for '//' itself,
because it would detect other syntax errors as well.

A flag meaning 'math error' (as on divide-by-zero or sqrt(-1))
might be useful too, I suppose....

--Matt