Remove dependence on Fortran for calculation of Bessel functions

[myurkin]

While compiling Fortran sources is not a big deal (can be done reliably in various environments), we have a special compilation option NO_FORTRAN to compile without them. The functionality of the obtained executable is naturally determined by the absence of the corresponding Fortran routines. The main dependence so far is IGT sources, which are especially important for rectangular sources. However, we will soon merge IGT_SO (#299), which provides an efficient substitute. After that IGT will be left mainly for testing purposes.

But simultaneously we are adding Bessel beams (#304) and later EELS (#155), which depend on calculation of Bessel functions (including modified ones of complex argument) in file bessel.f90. Thus, it would be nice to rewrite this file in C99 - I guess, this can be done semi-automatically.