The function simulates a Poisson sphere system of
intensity lam where each sphere center is uniformly
distributed in a box. The function returns a list of spheres with elements
id, center and radius r.
Any random generating function, passed as a name, for the radii distribution is accepted as long as
the formal function parameter names match the actual parameter names exactly as defined in
the parameter list theta.
The simulation box is of type list. The vector arguments correspond to the lower and upper points in x,y
and z direction. If box has only one element, i.e. list(c(0,1), the same extent is used for the other dimensions.
The argument pl denotes the print level of information during simulation. Currently, only
pl=0 for no output and pl>100 is implemented. Argument cond$rdist is of type string
naming the (user defined) radii random generating function.
Setting size equal to 'rlnorm' generates log normally distributed radii for a stationary Poisson
ball system according to a general approach of exact simulation (see reference below).