Reference frame for defining scattered angles with non-default incident direction

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Currently, we are working on DDA near surface, in which the laboratory 
reference frame (RF) will have larger emphasis, since it will be linked to the 
orientation of the surface. Therefore, we plan to define the scattering angles 
with respect to the laboratory RF, irrespective of the propagation direction 
(by the way, the default propagation direction is expected to be different too, 
along the _negative_ z-axis).

So, first, there will be different definitions of scattering angles for 
standard and near-surface modes. The idea of this issue is to merge them, i.e. 
to make the standard definition also with respect to the laboratory reference 
frame. However, there may be a lot of consequences of such changes (including, 
calculations of scat_grid and alldir), so a careful consideration is required. 
This may be connected to (simplified by) issue 138.

Original issue reported on code.google.com by yurkin on 6 Aug 2013 at 8:37