New ImageRec ctr that builds from two exemplars, with options for roi selection
IBA and oiiotool cleanup of add & sub
* Implement IBA::sub
* More robust handling of "full/display" rois in IBAprep
* Refactor oiiotool --add and --sub to allow different image sizes and
  handle pixel and display windows properly.
* Clarify comments
Tests for oiiotool --add and --sub
Merge pull request #572 from lgritz/lg-addsub

Add/subtract cleanup