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.

Passes are flipped when northbound, not southbound

I will tidy this up in the next few commits

Allows custom filenames for the output image
Fix formatting in help text

Add linear equalise
Add image flip for southbound passes
Completly remove old falseclor in favour of a much simplier method

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