Bugfix/fix asymptotic bessel

Makes the following corrections to multiprecision bessel function implementation:

• Include second series scaling with exp(-z) in asymptotic calculation of I
• Correct normalization of besinorm
• Make precision-dependent condition for selecting Taylor series or asymptotic approximation for I and K

The first change is the only one to affect the rest of the IW2D-scripts in their current form, as the normalized K and I are not used, and precision is hard coded. The first change is particularly important for small Re(z). All changes are based on the work of Amos: AMOS, D. E, Computation of Bessel functions of complex argument. SAND83-0086, Sandia National Laboratories, Albuquerque, N.M. (May 1983).