extract getErrorBuffer method to NcdOperationUtils
replace getErrorBuffer calls with static method

 * will square getError if only error is available
check for null loader
handle ROI better in sector integration

 * use current values in GUI instead of file values
 * check symmetry for validity
 * group adjustments together (average area, clipping compensation) for
   visibility
 * throw an exception if diffraction metadata is not available
extend SectorIntegrationModel instead of IntegrationModel

 * Jake suggests a no-argument constructor with a SectorROI, and this
   model already has this