add unit/integrted tests for dpc

I followed your suggestion to slice the result like that. I think think we should reserve the function for future improvement as the fitting plays a very important role in DPC efficiency and accuracy. 

> On Dec 5, 2014, at 1:55 PM, Thomas A Caswell <notifications@github.com> wrote:
> 
> In skxray/dpc.py:
> 
> > +    
> > +    tol : float, optional
> > +        Termination criteria of nonlinear fitting.
> > +    
> > +    max_iters : integer, optional
> > +        Maximum iterations of nonlinear fitting.
> > +    
> > +    Returns
> > +    -------
> > +    tuple 
> > +        Fitting result: intensity attenuation and phase gradient.
> > +    
> > +    """
> > +    
> > +    return minimize(rss, start_point, args=(ref_f, f), method=solver, tol=tol,
> > +                    options=dict(maxiter=max_iters)).x[:2]
> Why are you slicing the results like this?
> 
> I really don't think this function should exist, just use minimize directly in dpc_runner. That way you have access to the OptimizeResult which also includes things like goodness of fit measures.
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/Nikea/scikit-xray/pull/177/files#r21391440>.
> 

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Best Regards,

  Cheng Chang 
  Computer Engineering, Stony Brook University
  Computer Science Group, Computational Science Center, Brookhaven National Laboratory
  Tel：(001) 631-885-4922