Adding documentation for d_inner and d_outer calculation

This PR adds documentation for the d_inner and d_outer calculations in the code. This includes images in `png`-format as well as in the geogebra source format. http://www.geogebra.org/cms/en/ is free for many platforms and is an easy  way to create geometry graphics. 

Please check @ssim (or anyone else from @tardis-sn) and merge

@orbitfold this includes both descriptions for d_inner and d_outer routines. When re-writing them to C please update this documentation to tell which functions do that.