bessel_zeros: Finds the first n zeros of a bessel function
 	z = bessel_zeros(d, a, n, e)
 	z	=	zeros of the bessel function
 	d	=	Bessel function type:
 			1:	Ja
 			2:	Ya
 			3:	Ja'
 			4:	Ya'
 	a	=	Bessel order (a>=0)
 	n	=	Number of zeros to find
 	e	=	Relative error in root
 	This function uses the routine described in:
 		"An Algorithm with ALGOL 60 Program for the Computation of the
 		zeros of the Ordinary Bessel Functions and those of their
 		N. M. Temme
 		Journal of Computational Physics, 32, 270-279 (1979)