Radiative forces for particles near surface

[GoogleCodeExporter]

Make radiative forces computation for particles near surfaces. Probably, FFT 
implementation of standard radiative forces (issue 14) should first be made. 
But the sparse version is also viable, using a precomputed table of (modified) 
Sommerfeld integrals.

As a first step, it would also be nice to derive some simple formula to compute 
the total forces from some cross sections, at least in some special cases (see 
issue 133).

Original issue reported on code.google.com by yurkin on 25 Sep 2013 at 10:28

• Blocked on: #14

[myurkin]

#133