Optimize calculation of reflected Green's tensor

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Currently, somnec.c is used to compute a table of Sommerfeld integrals. There 
are two problems.

1) It is used as black box, without sufficient understanding of its accuracy. 
For some applications, the accuracy is probably not sufficient, for others - 
some of it can be sacrificed to get computational speed. This should be studied 
in more details (see comments in somnec.c). 

2) It takes considerable time, currently approximately equivalent to 200 
iterations. This can be prohibitive, e.g. for biological applications (large 
grid, but small number of iterations). So studying other alternatives 
(described in the beginning of issue 101) is still very relevant. However, the 
issue is not easy (preliminary work showed that it is not that easy to make a 
general "black box" routine with at least any automatic convergence control).

Alternative approach is to use 2D interpolating table to calculate Sommerfeld 
integrals - but that adds another free parameter, effecting the final accuracy.

Also, there is a connected (more specific) issue 175.

Original issue reported on code.google.com by yurkin on 25 Sep 2013 at 8:28

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We used the code from NEC2C from 
http://sharon.esrac.ele.tue.nl/users/pe1rxq/nec2c.rxq/nec2c.rxq-0.2.tar.gz. But 
later versions (1.3) are available at http://www.qsl.net/5b4az/ . It is not 
clear, however, if any changes to the Sommerfeld integrals part has been made.

Original comment by yurkin on 2 Feb 2014 at 9:33