Make the initial number of azimuthal grid points in near-to-far field transformation in cylindrical coordinates a user parameter

[oskooi]

Currently, the computation of the far fields in cylindrical coordinates involves an integration in $\phi$ (azimuthal angle) which is based on repeatedly doubling a grid of $\phi$ points using a hard-coded initial value:

meep/src/near2far.cpp

Lines 288 to 291 in 04fa305

	/* Perform phi integral. Since phi integrand is smooth, quadrature with equally spaced points
	should converge exponentially fast with the number N of quadrature points. We
	repeatedly double N until convergence to tol is achieved, re-using previous points. */
	const int N0 = 4;

The value of 4 turns out to be too small in some cases which produces discretization artifacts in the output as demonstrated in #2726 (comment). As such, it would be useful to make the initial number of $\phi$ grid points a user parameter in the routines get_farfield and get_farfields.