split cells to have headings in their own cell

plus add precision on convergence/consistency paragraph
Make text more precise when describing motion

especially when talking about the angles, we have pitch angle (positive
up), then glide angle in the force balance: a bit sloppy
turn around figure 3 for positive angle

and it's pitch angle, not glide angle
had to make a small change to reload fig

new figure not loading in nbviewer by simply replacing png file and
re-executing the markdown cell, yet git didn't detect a change in the
nb file?
minor change to line continuation suggestion
Merge pull request #10 from aitatanit/master

minor change to line continuation suggestion
change angle to gamma in force balance fig

there is still some confusion with angles in the text -- I updated the
figure showing forces to name the flight-path angle gamma (while
discussion above used theta for pitch angle, positive up!)
explain we follow Milne-Thompson/ centrifugal force

Derivation uses theta as positive upwards, and forces are in a rotating
frame, so we should properly speak of centrifugal force
checking a sign, executed style, no changes