print every quad in hex for ENHANCING readability

decimal could be read, but hex is perfect, since it allows to see
immediately in which level a presented quad lies
ADD particle printing function into simulation

every timestep we would like to see the actual positions of all our
particles
take care of particle ID!
let the simulation begin
substitute update_x by iterate updating each quad

going over all particles in all quads is most efficiently done via
p4est_iterate
add ToDo
simulate three static particles instead of a randome one
easy writing statistics to xyz or gnuplot file
scale down timestep .. it's still euler working here

to see a full cirle we need some more time
add another nice to have ToDo
every part of the code calling the particles ID needs to be in DEBUG

without DEBUG, there are no particle IDs
ADD particle_lies_in_quad() dummy
calculate the gradient of a circular velocity field around (0.5,0.5)
print particle locations in xyz or gnuplot format
move particles if they have left the quad
FIX presentation bug
too much DEBUG information
FIX bug FREEING IS NOT ITS RESPONSABILITY
replace dummy by efficient calculation
Merge branch 'movingThroughQuads'