
- pswarp takes an image (and corresponding mask and weight images) and
  warps the pixels to match the specified skycell projection and pixel
  grid.  the astrometry of the input image is supplied by the header
  of the image or by a specified SMP file. 

- the input image, weight, mask are loaded into pmFPA structures with
  the layout determined by the camera/format/layout

- the skycell is loaded into a pmFPA structure.  at a minimum, a
  single skycell may contain a single chip, cell, and readout, which
  in turn correspond to the single set of pixels representing the
  image region.  

  * is there a need or use for a skycell to have a multilayered
    structure equivalent in any way to the fpa->chip->cell->readout
    layer?

    - perhaps we will want to use a single fpa for a larger region
      with small sky cells representing contiguous pixel regions.  

    - perhaps we will want to warp an entire fpa exposure at once and
      use a skyfpa to represent all of the possible output skycells
      for a single input fpa exposure.

- the output is a file or set of files corresponding to the skycell
  files.  the details should be specified by the PSWARP.OUTPUT file
  rule.

