Exact Gaussian beam

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Currently, all beam options are only approximations, which may be
considerably inaccurate for tightly focused beams. Non-approximate
description exist in the literature, but it relies on elaborate
mathematics. The idea is to implement it in ADDA. 

Original issue reported on code.google.com by yurkin on 6 Nov 2009 at 9:42

[myurkin]

Current workaround for using exact Gaussian beams (or any other beam) is to calculate them on the grid outside of ADDA and use -beam read ...