Speedup corrector.cc

Add a specialization for the common case where the residual block
outputs exactly one residual.

The matrix routines used by Corrector can be then specialized to
a scalar and be made considerably faster.

For denoising upto 400% speedup is observed.

Change-Id: I8e3f24b8ba41caa8e62ad97c5f5e96ab6ea47150