add unit/integrted tests for dpc

This is because ‘_a' also provides some amplitude information about the sample. It’s free while computing the phase information, so we save it.

> On Dec 9, 2014, at 11:51 AM, Thomas A Caswell <notifications@github.com> wrote:
> 
> if a is not used, why are you collecting it?
> 
> 

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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