Improved thedocumentation and deprecated GenerateTheHills() in
vtkParametricRandomHills.

This change removes the need for the user to manually regenerate the
 centers of the hills, standard deviations and amplitudes whenever the
 parameters change.

Previously, if any parameters changed, then the user had to manually
 call GenerateTheHills() this meant that when using ParaView, only the
 number of hills could be changed. This change will allow all the
 parameters to be changed when using ParaView.

Change-Id: I6e8be04238591c8d435284e5eec2203ebd599dfa
Signed-off-by: Andrew Maclean <andrew.amaclean@gmail.com>