change default ngl
start on new documentation of 2d disk dfs
switch to default niter=20 for corrections (as in docs)
further re-arranging of the disk df documentation and description of moments using the corrected DFs
more docs on sampling the DF
move Dehnen example and other 'advanced disk DF' stuff to new 2d DF section
start on docs for the quasiisothermal DF