Subchapters:
 
Exercises programming
 Do a color plot of sin(x), cos(x), and tan(x)
 Do the same, but from a procedure with the xrange as parameter
 Do the same, but with xrange as a keyword and a default value
 Program a looping function for Factorielle(n)
 Program a recursive computation of Factorielle(n)
 Compare the times needed for the 2 previous computations
 Write a function that return the angular size in arcsec given a
distance in kpc and a projected distance in cm.
