Optical torque

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Currently ADDA doesn't calculate optical torques (either integral or 
dipole-wise). However, if the radiation forces are known on each dipole, the 
torque on each dipole can also be calculated, using Eq.(22) from
B. T. Draine and J. C. Weingartner, “Radiative torques on interstellar grains 
.1. Superthermal spin-up,” [http://dx.doi.org/10.1086/177887 Astrophys. J. 
470, 551–565 (1996)].
The problem is that the variable E_j in that formula is not the internal but 
rather exciting field (in ADDA terminology - see Section 9 "DDA Formulation" of 
the manual), which can be computed as alpha_j^(-1).P_j. alpha is shown in log 
file as "CoupleConstant" - either one value (scalar) or three values (diagonal 
tensor). P_j can be obtained from file produced by '-store_dip_pol'.

So a workaround to compute optical torque is to write down a simple script to 
process files RadForce, DipPol and log, as describe above. However, it should 
be even easier to implement this calculation inside ADDA.

Another approach is to calculate (only) the total torque through the far-field 
integration (described in the same reference above). This is related to issue 
133.

#133

Original issue reported on code.google.com by yurkin on 10 Nov 2013 at 11:46

[myurkin]

/cc @stefaniagl