Exported functions and types prefixed with apt_
  Callback added for getsample, instead of calling named function.
  Allow library to be built without libpng or libsndfile.
Add support for building with MSVC on Windows.
Fix a few MSVC compiler warnings.
Fix nonlinear radiance correction for NOAA19.

Improved commenting in dsp.c
Updated copyright notices
Code tidy up in some parts

new dsp code
ngvi test

change -s option