Compute locally distance to source in SRCALC

[srio]

There is an asymmetry in the power profile that may come from

1. The asymmetry of the source. This is considered by SRCALC
2. The asymmetry of the mirror profile (e.g., an ellipse). Again this is considered by SRCALC
3. The different angle of incidence at center and at the edges. This effect is not considered in SRCALC as it takes the average value. This effect is usually very small. It can be estimated with POWER, e.g., :
   Angle upstream, center, downstream: 0.0020134228098434024 0.002 0.0019865771901565977
   Power absorbed for angle upstream, center, downstream: 183.602697 183.523666 183.442252
   Relative power difference due to variation in incident angle -0.0874635 %
4. The mirror length, with edges that see difference distance to the source. This may be an important point. The pixel size changes with 1/(distance**2). An example, a mirror length of 400 mm at 30 m with 2 mrad we have 2.7% difference.This effect is easily corrected “a posteriori” in SRCALC results by multiplying by (distance^2)/(distance+x)^2. It would be nice to implement this effect in SRCALC. Check with Ruben if this was implemented in the original IGOR SRCALC.